diff --git a/tests/AbstractTestCase.php b/tests/AbstractTestCase.php
index 17f15dc..3ba6440 100644
--- a/tests/AbstractTestCase.php
+++ b/tests/AbstractTestCase.php
@@ -33,7 +33,7 @@ abstract class AbstractTestCase extends TestCase
      * @param string $expectedString The expected string representation.
      * @param object $object         The object to test.
      */
-    protected function assertIs(string $className, string $expectedString, $object)
+    protected function assertIs(string $className, string $expectedString, $object): void
     {
         $this->assertInstanceOf($className, $object);
         $this->assertSame($expectedString, (string) $object);
@@ -44,7 +44,7 @@ protected function assertIs(string $className, string $expectedString, $object)
      * @param int     $nano        The expected nanosecond adjustment.
      * @param Instant $instant     The instant to test.
      */
-    protected function assertInstantIs(int $epochSecond, int $nano, Instant $instant)
+    protected function assertInstantIs(int $epochSecond, int $nano, Instant $instant): void
     {
         $this->compare([$epochSecond, $nano], [
             $instant->getEpochSecond(),
@@ -58,7 +58,7 @@ protected function assertInstantIs(int $epochSecond, int $nano, Instant $instant
      * @param int       $day   The expected day.
      * @param LocalDate $date  The local date to test.
      */
-    protected function assertLocalDateIs(int $year, int $month, int $day, LocalDate $date)
+    protected function assertLocalDateIs(int $year, int $month, int $day, LocalDate $date): void
     {
         $this->compare([$year, $month, $day], [
             $date->getYear(),
@@ -74,7 +74,7 @@ protected function assertLocalDateIs(int $year, int $month, int $day, LocalDate
      * @param int       $nano   The expected nano-of-second.
      * @param LocalTime $time   The local time to test.
      */
-    protected function assertLocalTimeIs(int $hour, int $minute, int $second, int $nano, LocalTime $time)
+    protected function assertLocalTimeIs(int $hour, int $minute, int $second, int $nano, LocalTime $time): void
     {
         $this->compare([$hour, $minute, $second, $nano], [
             $time->getHour(),
@@ -94,7 +94,7 @@ protected function assertLocalTimeIs(int $hour, int $minute, int $second, int $n
      * @param int           $n        The expected nano-of-second.
      * @param LocalDateTime $dateTime The local date-time to test.
      */
-    protected function assertLocalDateTimeIs(int $y, int $m, int $d, int $h, int $i, int $s, int $n, LocalDateTime $dateTime)
+    protected function assertLocalDateTimeIs(int $y, int $m, int $d, int $h, int $i, int $s, int $n, LocalDateTime $dateTime): void
     {
         $this->compare([$y, $m, $d, $h, $i, $s, $n], [
             $dateTime->getYear(),
@@ -111,7 +111,7 @@ protected function assertLocalDateTimeIs(int $y, int $m, int $d, int $h, int $i,
      * @param LocalDateTime $expected The expected local date-time.
      * @param LocalDateTime $actual   The actual local date-time.
      */
-    protected function assertLocalDateTimeEquals(LocalDateTime $expected, LocalDateTime $actual)
+    protected function assertLocalDateTimeEquals(LocalDateTime $expected, LocalDateTime $actual): void
     {
         $this->assertTrue($actual->isEqualTo($expected), "$actual != $expected");
     }
@@ -120,7 +120,7 @@ protected function assertLocalDateTimeEquals(LocalDateTime $expected, LocalDateT
      * @param int  $yearValue
      * @param Year $year
      */
-    protected function assertYearIs(int $yearValue, Year $year)
+    protected function assertYearIs(int $yearValue, Year $year): void
     {
         $this->compare([$yearValue], [
             $year->getValue()
@@ -132,7 +132,7 @@ protected function assertYearIs(int $yearValue, Year $year)
      * @param int       $month     The expected month.
      * @param YearMonth $yearMonth The year-month to test.
      */
-    protected function assertYearMonthIs(int $year, int $month, YearMonth $yearMonth)
+    protected function assertYearMonthIs(int $year, int $month, YearMonth $yearMonth): void
     {
         $this->compare([$year, $month], [
             $yearMonth->getYear(),
@@ -145,7 +145,7 @@ protected function assertYearMonthIs(int $year, int $month, YearMonth $yearMonth
      * @param int      $week     The expected week.
      * @param YearWeek $yearWeek The year-week to test.
      */
-    protected function assertYearWeekIs(int $year, int $week, YearWeek $yearWeek)
+    protected function assertYearWeekIs(int $year, int $week, YearWeek $yearWeek): void
     {
         $this->compare([$year, $week], [
             $yearWeek->getYear(),
@@ -158,7 +158,7 @@ protected function assertYearWeekIs(int $year, int $week, YearWeek $yearWeek)
      * @param int      $day      The expected day.
      * @param MonthDay $monthDay The month-day to test.
      */
-    protected function assertMonthDayIs(int $month, int $day, MonthDay $monthDay)
+    protected function assertMonthDayIs(int $month, int $day, MonthDay $monthDay): void
     {
         $this->compare([$month, $day], [
             $monthDay->getMonth(),
@@ -170,7 +170,7 @@ protected function assertMonthDayIs(int $month, int $day, MonthDay $monthDay)
      * @param int   $monthValue The expected month-of-year value, from 1 to 12.
      * @param Month $month      The Month instance to test.
      */
-    protected function assertMonthIs(int $monthValue, Month $month)
+    protected function assertMonthIs(int $monthValue, Month $month): void
     {
         $this->compare([$monthValue], [
             $month->getValue()
@@ -181,7 +181,7 @@ protected function assertMonthIs(int $monthValue, Month $month)
      * @param int       $dayOfWeekValue The expected day-of-week value, from 1 to 7.
      * @param DayOfWeek $dayOfWeek      The DayOfWeek instance to test.
      */
-    protected function assertDayOfWeekIs(int $dayOfWeekValue, DayOfWeek $dayOfWeek)
+    protected function assertDayOfWeekIs(int $dayOfWeekValue, DayOfWeek $dayOfWeek): void
     {
         $this->compare([$dayOfWeekValue], [
             $dayOfWeek->getValue()
@@ -193,7 +193,7 @@ protected function assertDayOfWeekIs(int $dayOfWeekValue, DayOfWeek $dayOfWeek)
      * @param int      $nanos    The expected nanos.
      * @param Duration $duration The duration to test.
      */
-    protected function assertDurationIs(int $seconds, int $nanos, Duration $duration)
+    protected function assertDurationIs(int $seconds, int $nanos, Duration $duration): void
     {
         $this->compare([$seconds, $nanos], [
             $duration->getSeconds(),
@@ -207,7 +207,7 @@ protected function assertDurationIs(int $seconds, int $nanos, Duration $duration
      * @param int     $days   The expected number of days in the period.
      * @param Period  $period The period to test.
      */
-    protected function assertPeriodIs(int $years, int $months, int $days, Period $period)
+    protected function assertPeriodIs(int $years, int $months, int $days, Period $period): void
     {
         $this->compare([$years, $months, $days], [
             $period->getYears(),
@@ -225,7 +225,7 @@ protected function assertPeriodIs(int $years, int $months, int $days, Period $pe
      * @param int            $d2    The expected day-of-month of the end date.
      * @param LocalDateRange $range The LocalDateRange instance to test.
      */
-    protected function assertLocalDateRangeIs(int $y1, int $m1, int $d1, int $y2, int $m2, int $d2, LocalDateRange $range)
+    protected function assertLocalDateRangeIs(int $y1, int $m1, int $d1, int $y2, int $m2, int $d2, LocalDateRange $range): void
     {
         $this->assertLocalDateIs($y1, $m1, $d1, $range->getStart());
         $this->assertLocalDateIs($y2, $m2, $d2, $range->getEnd());
@@ -238,7 +238,7 @@ protected function assertLocalDateRangeIs(int $y1, int $m1, int $d1, int $y2, in
      * @param int            $m2    The expected month-of-year of the end year-month.
      * @param YearMonthRange $range The YearMonthRange instance to test.
      */
-    protected function assertYearMonthRangeIs(int $y1, int $m1, int $y2, int $m2, YearMonthRange $range)
+    protected function assertYearMonthRangeIs(int $y1, int $m1, int $y2, int $m2, YearMonthRange $range): void
     {
         $this->assertYearMonthIs($y1, $m1, $range->getStart());
         $this->assertYearMonthIs($y2, $m2, $range->getEnd());
@@ -248,7 +248,7 @@ protected function assertYearMonthRangeIs(int $y1, int $m1, int $y2, int $m2, Ye
      * @param TimeZone $expected The expected time-zone.
      * @param TimeZone $actual   The actual time-zone.
      */
-    protected function assertTimeZoneEquals(TimeZone $expected, TimeZone $actual)
+    protected function assertTimeZoneEquals(TimeZone $expected, TimeZone $actual): void
     {
         $this->assertTrue($actual->isEqualTo($expected), "$actual != $expected");
     }
@@ -257,7 +257,7 @@ protected function assertTimeZoneEquals(TimeZone $expected, TimeZone $actual)
      * @param int            $totalSeconds   The expected total offset in seconds.
      * @param TimeZoneOffset $timeZoneOffset The time-zone offset to test.
      */
-    protected function assertTimeZoneOffsetIs(int $totalSeconds, TimeZoneOffset $timeZoneOffset)
+    protected function assertTimeZoneOffsetIs(int $totalSeconds, TimeZoneOffset $timeZoneOffset): void
     {
         $this->compare([$totalSeconds], [
             $timeZoneOffset->getTotalSeconds()
@@ -268,7 +268,7 @@ protected function assertTimeZoneOffsetIs(int $totalSeconds, TimeZoneOffset $tim
      * @param array $expected The expected values.
      * @param array $actual   The actual values, count & keys matching expected values.
      */
-    private function compare(array $expected, array $actual)
+    private function compare(array $expected, array $actual): void
     {
         $message = $this->export($actual) . ' !== ' . $this->export($expected);
 
diff --git a/tests/Clock/FixedClockTest.php b/tests/Clock/FixedClockTest.php
index 2e5e6d0..352f21b 100644
--- a/tests/Clock/FixedClockTest.php
+++ b/tests/Clock/FixedClockTest.php
@@ -13,7 +13,7 @@
  */
 class FixedClockTest extends AbstractTestCase
 {
-    public function testFixedClock()
+    public function testFixedClock(): void
     {
         $clock = new FixedClock(Instant::of(123456789, 987654321));
         $this->assertInstantIs(123456789, 987654321, $clock->getTime());
diff --git a/tests/Clock/OffsetClockTest.php b/tests/Clock/OffsetClockTest.php
index 0baa497..f5ab38e 100644
--- a/tests/Clock/OffsetClockTest.php
+++ b/tests/Clock/OffsetClockTest.php
@@ -24,7 +24,7 @@ class OffsetClockTest extends AbstractTestCase
      * @param int    $expectedSecond The expected epoch second returned by the clock.
      * @param int    $expectedNano   The expected nano returned by the clock.
      */
-    public function testOffsetClock(int $second, int $nano, string $duration, int $expectedSecond, int $expectedNano)
+    public function testOffsetClock(int $second, int $nano, string $duration, int $expectedSecond, int $expectedNano): void
     {
         $baseClock = new FixedClock(Instant::of($second, $nano));
         $clock = new OffsetClock($baseClock, Duration::parse($duration));
@@ -32,9 +32,6 @@ public function testOffsetClock(int $second, int $nano, string $duration, int $e
         $this->assertInstantIs($expectedSecond, $expectedNano, $clock->getTime());
     }
 
-    /**
-     * @return array
-     */
     public function providerOffsetClock() : array
     {
         return [
diff --git a/tests/Clock/ScaleClockTest.php b/tests/Clock/ScaleClockTest.php
index dd2d1a6..6c0abe5 100644
--- a/tests/Clock/ScaleClockTest.php
+++ b/tests/Clock/ScaleClockTest.php
@@ -24,7 +24,7 @@ class ScaleClockTest extends AbstractTestCase
      * @param int    $scale           The time scale.
      * @param string $expectedInstant The expected epoch second returned by the clock.
      */
-    public function testScaleClock(int $second, int $nano, string $duration, int $scale, string $expectedInstant)
+    public function testScaleClock(int $second, int $nano, string $duration, int $scale, string $expectedInstant): void
     {
         $baseInstant = Instant::of($second, $nano);
 
@@ -39,9 +39,6 @@ public function testScaleClock(int $second, int $nano, string $duration, int $sc
         $this->assertSame($expectedInstant, $actualTime->toDecimal());
     }
 
-    /**
-     * @return array
-     */
     public function providerScaleClock() : array
     {
         return [
diff --git a/tests/Clock/SystemClockTest.php b/tests/Clock/SystemClockTest.php
index 8945e51..5cec704 100644
--- a/tests/Clock/SystemClockTest.php
+++ b/tests/Clock/SystemClockTest.php
@@ -7,10 +7,8 @@
     /**
      * Re-declare microtime() in the namespace of the SystemClock,
      * to trick it into thinking it's the native PHP function.
-     *
-     * @return string
      */
-    function microtime() {
+    function microtime(): string {
         return '0.55527600 14079491701';
     }
 }
@@ -25,7 +23,7 @@ function microtime() {
      */
     class SystemClockTest extends AbstractTestCase
     {
-        public function testSystemClock()
+        public function testSystemClock(): void
         {
             $clock = new SystemClock();
 
diff --git a/tests/DayOfWeekTest.php b/tests/DayOfWeekTest.php
index 8f331b7..3d5e48b 100644
--- a/tests/DayOfWeekTest.php
+++ b/tests/DayOfWeekTest.php
@@ -22,14 +22,11 @@ class DayOfWeekTest extends AbstractTestCase
      * @param int $expectedValue     The expected value of the constant.
      * @param int $dayOfWeekConstant The day-of-week constant.
      */
-    public function testConstants(int $expectedValue, int $dayOfWeekConstant)
+    public function testConstants(int $expectedValue, int $dayOfWeekConstant): void
     {
         $this->assertSame($expectedValue, $dayOfWeekConstant);
     }
 
-    /**
-     * @return array
-     */
     public function providerConstants() : array
     {
         return [
@@ -43,25 +40,20 @@ public function providerConstants() : array
         ];
     }
 
-    public function testOf()
+    public function testOf(): void
     {
         $this->assertDayOfWeekIs(5, DayOfWeek::of(5));
     }
 
     /**
      * @dataProvider providerOfInvalidDayOfWeekThrowsException
-     *
-     * @param int $dayOfWeek
      */
-    public function testOfInvalidDayOfWeekThrowsException(int $dayOfWeek)
+    public function testOfInvalidDayOfWeekThrowsException(int $dayOfWeek): void
     {
         $this->expectException(DateTimeException::class);
         DayOfWeek::of($dayOfWeek);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInvalidDayOfWeekThrowsException() : array
     {
         return [
@@ -78,15 +70,12 @@ public function providerOfInvalidDayOfWeekThrowsException() : array
      * @param string $timeZone          The time-zone to get the current day-of-week in.
      * @param int    $expectedDayOfWeek The expected day-of-week, from 1 to 7.
      */
-    public function testNow(int $epochSecond, string $timeZone, int $expectedDayOfWeek)
+    public function testNow(int $epochSecond, string $timeZone, int $expectedDayOfWeek): void
     {
         $clock = new FixedClock(Instant::of($epochSecond));
         $this->assertDayOfWeekIs($expectedDayOfWeek, DayOfWeek::now(TimeZone::parse($timeZone), $clock));
     }
 
-    /**
-     * @return array
-     */
     public function providerNow() : array
     {
         return [
@@ -99,7 +88,7 @@ public function providerNow() : array
         ];
     }
 
-    public function testAll()
+    public function testAll(): void
     {
         for ($day = DayOfWeek::MONDAY; $day <= DayOfWeek::SUNDAY; $day++) {
             $dayOfWeek = DayOfWeek::of($day);
@@ -111,42 +100,42 @@ public function testAll()
         }
     }
 
-    public function testMonday()
+    public function testMonday(): void
     {
         $this->assertDayOfWeekIs(DayOfWeek::MONDAY, DayOfWeek::monday());
     }
 
-    public function testTuesday()
+    public function testTuesday(): void
     {
         $this->assertDayOfWeekIs(DayOfWeek::TUESDAY, DayOfWeek::tuesday());
     }
 
-    public function testWednesday()
+    public function testWednesday(): void
     {
         $this->assertDayOfWeekIs(DayOfWeek::WEDNESDAY, DayOfWeek::wednesday());
     }
 
-    public function testThursday()
+    public function testThursday(): void
     {
         $this->assertDayOfWeekIs(DayOfWeek::THURSDAY, DayOfWeek::thursday());
     }
 
-    public function testFriday()
+    public function testFriday(): void
     {
         $this->assertDayOfWeekIs(DayOfWeek::FRIDAY, DayOfWeek::friday());
     }
 
-    public function testSaturday()
+    public function testSaturday(): void
     {
         $this->assertDayOfWeekIs(DayOfWeek::SATURDAY, DayOfWeek::saturday());
     }
 
-    public function testSunday()
+    public function testSunday(): void
     {
         $this->assertDayOfWeekIs(DayOfWeek::SUNDAY, DayOfWeek::sunday());
     }
 
-    public function testIs()
+    public function testIs(): void
     {
         for ($i = DayOfWeek::MONDAY; $i <= DayOfWeek::SUNDAY; $i++) {
             for ($j = DayOfWeek::MONDAY; $j <= DayOfWeek::SUNDAY; $j++) {
@@ -155,7 +144,7 @@ public function testIs()
         }
     }
 
-    public function testIsEqualTo()
+    public function testIsEqualTo(): void
     {
         for ($i = DayOfWeek::MONDAY; $i <= DayOfWeek::SUNDAY; $i++) {
             for ($j = DayOfWeek::MONDAY; $j <= DayOfWeek::SUNDAY; $j++) {
@@ -171,7 +160,7 @@ public function testIsEqualTo()
      * @param int $plusDays          The number of days to add.
      * @param int $expectedDayOfWeek The expected day-of-week value, from 1 to 7.
      */
-    public function testPlus(int $dayOfWeek, int $plusDays, int $expectedDayOfWeek)
+    public function testPlus(int $dayOfWeek, int $plusDays, int $expectedDayOfWeek): void
     {
         $this->assertDayOfWeekIs($expectedDayOfWeek, DayOfWeek::of($dayOfWeek)->plus($plusDays));
     }
@@ -183,14 +172,11 @@ public function testPlus(int $dayOfWeek, int $plusDays, int $expectedDayOfWeek)
      * @param int $plusDays          The number of days to add.
      * @param int $expectedDayOfWeek The expected day-of-week value, from 1 to 7.
      */
-    public function testMinus(int $dayOfWeek, int $plusDays, int $expectedDayOfWeek)
+    public function testMinus(int $dayOfWeek, int $plusDays, int $expectedDayOfWeek): void
     {
         $this->assertDayOfWeekIs($expectedDayOfWeek, DayOfWeek::of($dayOfWeek)->minus(-$plusDays));
     }
 
-    /**
-     * @return \Generator
-     */
     public function providerPlus() : \Generator
     {
         for ($dayOfWeek = DayOfWeek::MONDAY; $dayOfWeek <= DayOfWeek::SUNDAY; $dayOfWeek++) {
@@ -217,7 +203,7 @@ public function providerPlus() : \Generator
      * @param string $localDate The local date to test, as a string.
      * @param int    $dayOfWeek The day-of-week number that matches the local date.
      */
-    public function testGetDayOfWeekFromLocalDate(string $localDate, int $dayOfWeek)
+    public function testGetDayOfWeekFromLocalDate(string $localDate, int $dayOfWeek): void
     {
         $localDate = LocalDate::parse($localDate);
         $dayOfWeek = DayOfWeek::of($dayOfWeek);
@@ -225,9 +211,6 @@ public function testGetDayOfWeekFromLocalDate(string $localDate, int $dayOfWeek)
         $this->assertTrue($localDate->getDayOfWeek()->isEqualTo($dayOfWeek));
     }
 
-    /**
-     * @return array
-     */
     public function providerGetDayOfWeekFromLocalDate() : array
     {
         return [
@@ -253,7 +236,7 @@ public function providerGetDayOfWeekFromLocalDate() : array
      * @param int    $dayOfWeek    The day-of-week value, from 1 to 7.
      * @param string $expectedName The expected name.
      */
-    public function testJsonSerialize(int $dayOfWeek, string $expectedName)
+    public function testJsonSerialize(int $dayOfWeek, string $expectedName): void
     {
         $this->assertSame(json_encode($expectedName), json_encode(DayOfWeek::of($dayOfWeek)));
     }
@@ -264,7 +247,7 @@ public function testJsonSerialize(int $dayOfWeek, string $expectedName)
      * @param int    $dayOfWeek    The day-of-week value, from 1 to 7.
      * @param string $expectedName The expected name.
      */
-    public function testToString(int $dayOfWeek, string $expectedName)
+    public function testToString(int $dayOfWeek, string $expectedName): void
     {
         $this->assertSame($expectedName, (string) DayOfWeek::of($dayOfWeek));
     }
@@ -284,5 +267,4 @@ public function providerToString() : array
             [DayOfWeek::SUNDAY,    'Sunday']
         ];
     }
-
 }
diff --git a/tests/DefaultClockTest.php b/tests/DefaultClockTest.php
index 4e8b483..b524a82 100644
--- a/tests/DefaultClockTest.php
+++ b/tests/DefaultClockTest.php
@@ -18,13 +18,13 @@ public function tearDown(): void
         DefaultClock::reset();
     }
 
-    public function testFreeze()
+    public function testFreeze(): void
     {
         DefaultClock::freeze(Instant::of(123456, 5000));
         $this->assertInstantIs(123456, 5000, Instant::now());
     }
 
-    public function testTravel()
+    public function testTravel(): void
     {
         $fixedClock = new FixedClock(Instant::of(1000, 0));
         DefaultClock::set($fixedClock);
@@ -37,7 +37,7 @@ public function testTravel()
         $this->assertInstantIs(-998, 0, Instant::now());
     }
 
-    public function testScale()
+    public function testScale(): void
     {
         $fixedClock = new FixedClock(Instant::of(1000, 0));
         DefaultClock::set($fixedClock);
diff --git a/tests/DurationTest.php b/tests/DurationTest.php
index 9697b47..4bbea3d 100644
--- a/tests/DurationTest.php
+++ b/tests/DurationTest.php
@@ -15,7 +15,7 @@
  */
 class DurationTest extends AbstractTestCase
 {
-    public function testZero()
+    public function testZero(): void
     {
         $this->assertDurationIs(0, 0, Duration::zero());
     }
@@ -24,19 +24,16 @@ public function testZero()
      * @dataProvider providerOfSeconds
      *
      * @param int $seconds         The duration in seconds.
-     * @param int $nanoAdjustment  The nanoseconds adjustement to the duration.
+     * @param int $nanoAdjustment  The nanoseconds adjustment to the duration.
      * @param int $expectedSeconds The expected adjusted duration seconds.
      * @param int $expectedNanos   The expected adjusted duration nanoseconds.
      */
-    public function testOfSeconds(int $seconds, int $nanoAdjustment, int $expectedSeconds, int $expectedNanos)
+    public function testOfSeconds(int $seconds, int $nanoAdjustment, int $expectedSeconds, int $expectedNanos): void
     {
         $duration = Duration::ofSeconds($seconds, $nanoAdjustment);
         $this->assertDurationIs($expectedSeconds, $expectedNanos, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfSeconds() : array
     {
         return [
@@ -52,19 +49,15 @@ public function providerOfSeconds() : array
     }
 
     /**
-     * @param int $millis
      * @dataProvider providerOfMillis
      */
-    public function testOfMillis(int $millis)
+    public function testOfMillis(int $millis): void
     {
         $duration = Duration::ofMillis($millis);
         $this->assertSame($millis, $duration->getTotalMillis());
     }
 
-    /**
-     * @return array
-     */
-    public function providerOfMillis()
+    public function providerOfMillis(): array
     {
         return [
             [1000],
@@ -77,20 +70,13 @@ public function providerOfMillis()
 
     /**
      * @dataProvider providerOfNanos
-     *
-     * @param int $nanos
-     * @param int $expectedSeconds
-     * @param int $expectedNanos
      */
-    public function testOfNanos(int $nanos, int $expectedSeconds, int $expectedNanos)
+    public function testOfNanos(int $nanos, int $expectedSeconds, int $expectedNanos): void
     {
         $duration = Duration::ofNanos($nanos);
         $this->assertDurationIs($expectedSeconds, $expectedNanos, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfNanos() : array
     {
         return [
@@ -102,21 +88,21 @@ public function providerOfNanos() : array
         ];
     }
 
-    public function testOfMinutes()
+    public function testOfMinutes(): void
     {
         for ($i = -2; $i <= 2; $i++) {
             $this->assertSame($i * 60, Duration::ofMinutes($i)->getSeconds());
         }
     }
 
-    public function testOfHours()
+    public function testOfHours(): void
     {
         for ($i = -2; $i <= 2; $i++) {
             $this->assertSame($i * 3600, Duration::ofHours($i)->getSeconds());
         }
     }
 
-    public function testOfDays()
+    public function testOfDays(): void
     {
         for ($i = -2; $i <= 2; $i++) {
             $this->assertSame($i * 86400, Duration::ofDays($i)->getSeconds());
@@ -125,15 +111,8 @@ public function testOfDays()
 
     /**
      * @dataProvider providerBetween
-     *
-     * @param int $seconds1
-     * @param int $nanos1
-     * @param int $seconds2
-     * @param int $nanos2
-     * @param int $seconds
-     * @param int $nanos
      */
-    public function testBetween(int $seconds1, int $nanos1, int $seconds2, int $nanos2, int $seconds, int $nanos)
+    public function testBetween(int $seconds1, int $nanos1, int $seconds2, int $nanos2, int $seconds, int $nanos): void
     {
         $i1 = Instant::of($seconds1, $nanos1);
         $i2 = Instant::of($seconds2, $nanos2);
@@ -141,9 +120,6 @@ public function testBetween(int $seconds1, int $nanos1, int $seconds2, int $nano
         $this->assertDurationIs($seconds, $nanos, Duration::between($i1, $i2));
     }
 
-    /**
-     * @return array
-     */
     public function providerBetween() : array
     {
         return [
@@ -172,14 +148,11 @@ public function providerBetween() : array
      * @param int    $seconds The expected seconds.
      * @param int    $nanos   The expected nanos.
      */
-    public function testParse(string $text, int $seconds, int $nanos)
+    public function testParse(string $text, int $seconds, int $nanos): void
     {
         $this->assertDurationIs($seconds, $nanos, Duration::parse($text));
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -273,15 +246,12 @@ public function providerParse() : array
      *
      * @param string $text The string to test.
      */
-    public function testParseFailureThrowsException(string $text)
+    public function testParseFailureThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         Duration::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseFailureThrowsException() : array
     {
         return [
@@ -328,7 +298,7 @@ public function providerParseFailureThrowsException() : array
      * @param int $nanos   The nanos of the duration.
      * @param int $cmp     The comparison value.
      */
-    public function testIsZero(int $seconds, int $nanos, int $cmp)
+    public function testIsZero(int $seconds, int $nanos, int $cmp): void
     {
         $this->assertSame($cmp === 0, Duration::ofSeconds($seconds, $nanos)->isZero());
     }
@@ -340,7 +310,7 @@ public function testIsZero(int $seconds, int $nanos, int $cmp)
      * @param int $nanos   The nanos of the duration.
      * @param int $cmp     The comparison value.
      */
-    public function testIsPositive(int $seconds, int $nanos, int $cmp)
+    public function testIsPositive(int $seconds, int $nanos, int $cmp): void
     {
         $this->assertSame($cmp > 0, Duration::ofSeconds($seconds, $nanos)->isPositive());
     }
@@ -352,7 +322,7 @@ public function testIsPositive(int $seconds, int $nanos, int $cmp)
      * @param int $nanos   The nanos of the duration.
      * @param int $cmp     The comparison value.
      */
-    public function testIsPositiveOrZero(int $seconds, int $nanos, int $cmp)
+    public function testIsPositiveOrZero(int $seconds, int $nanos, int $cmp): void
     {
         $this->assertSame($cmp >= 0, Duration::ofSeconds($seconds, $nanos)->isPositiveOrZero());
     }
@@ -364,7 +334,7 @@ public function testIsPositiveOrZero(int $seconds, int $nanos, int $cmp)
      * @param int $nanos   The nanos of the duration.
      * @param int $cmp     The comparison value.
      */
-    public function testIsNegative(int $seconds, int $nanos, int $cmp)
+    public function testIsNegative(int $seconds, int $nanos, int $cmp): void
     {
         $this->assertSame($cmp < 0, Duration::ofSeconds($seconds, $nanos)->isNegative());
     }
@@ -376,14 +346,11 @@ public function testIsNegative(int $seconds, int $nanos, int $cmp)
      * @param int $nanos   The nanos of the duration.
      * @param int $cmp     The comparison value.
      */
-    public function testIsNegativeOrZero(int $seconds, int $nanos, int $cmp)
+    public function testIsNegativeOrZero(int $seconds, int $nanos, int $cmp): void
     {
         $this->assertSame($cmp <= 0, Duration::ofSeconds($seconds, $nanos)->isNegativeOrZero());
     }
 
-    /**
-     * @return array
-     */
     public function providerCompareToZero() : array
     {
         return [
@@ -408,7 +375,7 @@ public function providerCompareToZero() : array
      * @param int $nanos2   The nanoseconds of the 2nd duration.
      * @param int $expected The expected return value.
      */
-    public function testCompareTo(int $seconds1, int $nanos1, int $seconds2, int $nanos2, int $expected)
+    public function testCompareTo(int $seconds1, int $nanos1, int $seconds2, int $nanos2, int $expected): void
     {
         $duration1 = Duration::ofSeconds($seconds1, $nanos1);
         $duration2 = Duration::ofSeconds($seconds2, $nanos2);
@@ -416,9 +383,6 @@ public function testCompareTo(int $seconds1, int $nanos1, int $seconds2, int $na
         $this->assertSame($expected, $duration1->compareTo($duration2));
     }
 
-    /**
-     * @return array
-     */
     public function providerCompareTo() : array
     {
         return [
@@ -484,7 +448,7 @@ public function providerCompareTo() : array
      * @param int $s  The expected seconds.
      * @param int $n  The expected nanoseconds.
      */
-    public function testPlus(int $s1, int $n1, int $s2, int $n2, int $s, int $n)
+    public function testPlus(int $s1, int $n1, int $s2, int $n2, int $s, int $n): void
     {
         $duration1 = Duration::ofSeconds($s1, $n1);
         $duration2 = Duration::ofSeconds($s2, $n2);
@@ -502,7 +466,7 @@ public function testPlus(int $s1, int $n1, int $s2, int $n2, int $s, int $n)
      * @param int $s  The expected seconds.
      * @param int $n  The expected nanoseconds.
      */
-    public function testMinus(int $s1, int $n1, int $s2, int $n2, int $s, int $n)
+    public function testMinus(int $s1, int $n1, int $s2, int $n2, int $s, int $n): void
     {
         $duration1 = Duration::ofSeconds($s1, $n1);
         $duration2 = Duration::ofSeconds(-$s2, -$n2);
@@ -510,9 +474,6 @@ public function testMinus(int $s1, int $n1, int $s2, int $n2, int $s, int $n)
         $this->assertDurationIs($s, $n, $duration1->minus($duration2));
     }
 
-    /**
-     * @return array
-     */
     public function providerPlus() : array
     {
         return [
@@ -575,22 +536,13 @@ public function providerPlus() : array
 
     /**
      * @dataProvider providerPlusSeconds
-     *
-     * @param int $seconds
-     * @param int $nanos
-     * @param int $secondsToAdd
-     * @param int $expectedSeconds
-     * @param int $expectedNanos
      */
-    public function testPlusSeconds(int $seconds, int $nanos, int $secondsToAdd, int $expectedSeconds, int $expectedNanos)
+    public function testPlusSeconds(int $seconds, int $nanos, int $secondsToAdd, int $expectedSeconds, int $expectedNanos): void
     {
         $duration = Duration::ofSeconds($seconds, $nanos)->plusSeconds($secondsToAdd);
         $this->assertDurationIs($expectedSeconds, $expectedNanos, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusSeconds() : array
     {
         return [
@@ -630,9 +582,6 @@ public function testPlusMinutes(int $seconds, int $minutesToAdd, int $expectedSe
         $this->assertDurationIs($expectedSeconds, 0, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusMinutes() : array
     {
         return [
@@ -654,20 +603,13 @@ public function providerPlusMinutes() : array
 
     /**
      * @dataProvider providerPlusHours
-     *
-     * @param int $seconds
-     * @param int $hoursToAdd
-     * @param int $expectedSeconds
      */
-    public function testPlusHours(int $seconds, int $hoursToAdd, int $expectedSeconds)
+    public function testPlusHours(int $seconds, int $hoursToAdd, int $expectedSeconds): void
     {
         $duration = Duration::ofSeconds($seconds)->plusHours($hoursToAdd);
         $this->assertDurationIs($expectedSeconds, 0, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusHours() : array
     {
         return [
@@ -689,20 +631,13 @@ public function providerPlusHours() : array
 
     /**
      * @dataProvider providerPlusDays
-     *
-     * @param int $seconds
-     * @param int $daysToAdd
-     * @param int $expectedSeconds
      */
-    public function testPlusDays(int $seconds, int $daysToAdd, int $expectedSeconds)
+    public function testPlusDays(int $seconds, int $daysToAdd, int $expectedSeconds): void
     {
         $duration = Duration::ofSeconds($seconds)->plusDays($daysToAdd);
         $this->assertDurationIs($expectedSeconds, 0, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusDays() : array
     {
         return [
@@ -724,20 +659,13 @@ public function providerPlusDays() : array
 
     /**
      * @dataProvider providerMinusSeconds
-     *
-     * @param int $seconds
-     * @param int $secondsToSubtract
-     * @param int $expectedSeconds
      */
-    public function testMinusSeconds(int $seconds, int $secondsToSubtract, int $expectedSeconds)
+    public function testMinusSeconds(int $seconds, int $secondsToSubtract, int $expectedSeconds): void
     {
         $duration = Duration::ofSeconds($seconds)->minusSeconds($secondsToSubtract);
         $this->assertDurationIs($expectedSeconds, 0, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusSeconds() : array
     {
         return [
@@ -762,20 +690,13 @@ public function providerMinusSeconds() : array
 
     /**
      * @dataProvider providerMinusMinutes
-     *
-     * @param int $seconds
-     * @param int $minutesToSubtract
-     * @param int $expectedSeconds
      */
-    public function testMinusMinutes(int $seconds, int $minutesToSubtract, int $expectedSeconds)
+    public function testMinusMinutes(int $seconds, int $minutesToSubtract, int $expectedSeconds): void
     {
         $duration = Duration::ofSeconds($seconds)->minusMinutes($minutesToSubtract);
         $this->assertDurationIs($expectedSeconds, 0, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusMinutes() : array
     {
         return [
@@ -797,20 +718,13 @@ public function providerMinusMinutes() : array
 
     /**
      * @dataProvider providerMinusHours
-     *
-     * @param int $seconds
-     * @param int $hoursToSubtract
-     * @param int $expectedSeconds
      */
-    public function testMinusHours(int $seconds, int $hoursToSubtract, int $expectedSeconds)
+    public function testMinusHours(int $seconds, int $hoursToSubtract, int $expectedSeconds): void
     {
         $duration = Duration::ofSeconds($seconds)->minusHours($hoursToSubtract);
         $this->assertDurationIs($expectedSeconds, 0, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusHours() : array
     {
         return [
@@ -832,20 +746,13 @@ public function providerMinusHours() : array
 
     /**
      * @dataProvider providerMinusDays
-     *
-     * @param int $seconds
-     * @param int $daysToSubtract
-     * @param int $expectedSeconds
      */
-    public function testMinusDays(int $seconds, int $daysToSubtract, int $expectedSeconds)
+    public function testMinusDays(int $seconds, int $daysToSubtract, int $expectedSeconds): void
     {
         $duration = Duration::ofSeconds($seconds)->minusDays($daysToSubtract);
         $this->assertDurationIs($expectedSeconds, 0, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusDays() : array
     {
         return [
@@ -867,14 +774,8 @@ public function providerMinusDays() : array
 
     /**
      * @dataProvider providerMultipliedBy
-     *
-     * @param int $second
-     * @param int $nano
-     * @param int $multiplicand
-     * @param int $expectedSecond
-     * @param int $expectedNano
      */
-    public function testMultipliedBy(int $second, int $nano, int $multiplicand, int $expectedSecond, int $expectedNano)
+    public function testMultipliedBy(int $second, int $nano, int $multiplicand, int $expectedSecond, int $expectedNano): void
     {
         $duration = Duration::ofSeconds($second, $nano);
         $duration = $duration->multipliedBy($multiplicand);
@@ -882,9 +783,6 @@ public function testMultipliedBy(int $second, int $nano, int $multiplicand, int
         $this->assertDurationIs($expectedSecond, $expectedNano, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerMultipliedBy() : array
     {
         return [
@@ -970,22 +868,13 @@ public function providerMultipliedBy() : array
 
     /**
      * @dataProvider providerDividedBy
-     *
-     * @param int $seconds
-     * @param int $nanos
-     * @param int $divisor
-     * @param int $expectedSeconds
-     * @param int $expectedNanos
      */
-    public function testDividedBy(int $seconds, int $nanos, int $divisor, int $expectedSeconds, int $expectedNanos)
+    public function testDividedBy(int $seconds, int $nanos, int $divisor, int $expectedSeconds, int $expectedNanos): void
     {
         $duration = Duration::ofSeconds($seconds, $nanos)->dividedBy($divisor);
         $this->assertDurationIs($expectedSeconds, $expectedNanos, $duration);
     }
 
-    /**
-     * @return array
-     */
     public function providerDividedBy() : array
     {
         return [
@@ -1042,7 +931,7 @@ public function providerDividedBy() : array
         ];
     }
 
-    public function testDividedByZeroThrowsException()
+    public function testDividedByZeroThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
         Duration::zero()->dividedBy(0);
@@ -1052,19 +941,16 @@ public function testDividedByZeroThrowsException()
      * @dataProvider providerNegated
      *
      * @param int $seconds         The duration in seconds.
-     * @param int $nanos           The nanoseconds adjustement to the duration.
+     * @param int $nanos           The nanoseconds adjustment to the duration.
      * @param int $expectedSeconds The expected seconds of the negated duration.
      * @param int $expectedNanos   The expected nanoseconds adjustment of the negated duration.
      */
-    public function testNegated(int $seconds, int $nanos, int $expectedSeconds, int $expectedNanos)
+    public function testNegated(int $seconds, int $nanos, int $expectedSeconds, int $expectedNanos): void
     {
         $duration = Duration::ofSeconds($seconds, $nanos);
         $this->assertDurationIs($expectedSeconds, $expectedNanos, $duration->negated());
     }
 
-    /**
-     * @return array
-     */
     public function providerNegated() : array
     {
         return [
@@ -1078,7 +964,7 @@ public function providerNegated() : array
         ];
     }
 
-    public function testAbs()
+    public function testAbs(): void
     {
         for ($seconds = -3; $seconds <= 3; $seconds++) {
             $duration = Duration::ofSeconds($seconds)->abs();
@@ -1086,7 +972,7 @@ public function testAbs()
         }
     }
 
-    public function testComparisons()
+    public function testComparisons(): void
     {
         $this->doTestComparisons([
             Duration::ofDays(-1),
@@ -1110,7 +996,7 @@ public function testComparisons()
     /**
      * @param Duration[] $durations
      */
-    private function doTestComparisons(array $durations)
+    private function doTestComparisons(array $durations): void
     {
         $count = \count($durations);
 
@@ -1149,7 +1035,7 @@ private function doTestComparisons(array $durations)
         }
     }
 
-    public function testEquals()
+    public function testEquals(): void
     {
         $test5a = Duration::ofSeconds(5);
         $test5b = Duration::ofSeconds(5);
@@ -1184,15 +1070,12 @@ public function testEquals()
      * @param int $nanos          The nanoseconds adjustment to the duration.
      * @param int $expectedMillis The expected total number of milliseconds.
      */
-    public function testGetTotalMillis(int $seconds, int $nanos, int $expectedMillis)
+    public function testGetTotalMillis(int $seconds, int $nanos, int $expectedMillis): void
     {
         $duration = Duration::ofSeconds($seconds, $nanos);
         $this->assertSame($expectedMillis, $duration->getTotalMillis());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetTotalMillis() : array
     {
         return [
@@ -1210,15 +1093,12 @@ public function providerGetTotalMillis() : array
      * @param int $nanos          The nanoseconds adjustment to the duration.
      * @param int $expectedMicros The expected total number of microseconds.
      */
-    public function testGetTotalMicros(int $seconds, int $nanos, int $expectedMicros)
+    public function testGetTotalMicros(int $seconds, int $nanos, int $expectedMicros): void
     {
         $duration = Duration::ofSeconds($seconds, $nanos);
         $this->assertSame($expectedMicros, $duration->getTotalMicros());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetTotalMicros() : array
     {
         return [
@@ -1236,15 +1116,12 @@ public function providerGetTotalMicros() : array
      * @param int $nanos         The nanoseconds adjustment to the duration.
      * @param int $expectedNanos The expected total number of nanoseconds.
      */
-    public function testGetTotalNanos(int $seconds, int $nanos, int $expectedNanos)
+    public function testGetTotalNanos(int $seconds, int $nanos, int $expectedNanos): void
     {
         $duration = Duration::ofSeconds($seconds, $nanos);
         $this->assertSame($expectedNanos, $duration->getTotalNanos());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetTotalNanos() : array
     {
         return [
@@ -1257,18 +1134,12 @@ public function providerGetTotalNanos() : array
 
     /**
      * @dataProvider providerToDaysPart
-     *
-     * @param Duration $duration
-     * @param int      $days
      */
-    public function testToDaysPart(Duration $duration, int $days)
+    public function testToDaysPart(Duration $duration, int $days): void
     {
         $this->assertSame($days, $duration->toDaysPart());
     }
 
-    /**
-     * @return array
-     */
     public function providerToDaysPart() : array
     {
         return [
@@ -1283,18 +1154,12 @@ public function providerToDaysPart() : array
 
     /**
      * @dataProvider providerToHoursPart
-     *
-     * @param Duration $duration
-     * @param int      $hours
      */
-    public function testToHoursPart(Duration $duration, int $hours)
+    public function testToHoursPart(Duration $duration, int $hours): void
     {
         $this->assertSame($hours, $duration->toHoursPart());
     }
 
-    /**
-     * @return array
-     */
     public function providerToHoursPart() : array
     {
         return [
@@ -1308,18 +1173,12 @@ public function providerToHoursPart() : array
 
     /**
      * @dataProvider providerToMinutesPart
-     *
-     * @param Duration $duration
-     * @param int      $minutes
      */
-    public function testToMinutesPart(Duration $duration, int $minutes)
+    public function testToMinutesPart(Duration $duration, int $minutes): void
     {
         $this->assertSame($minutes, $duration->toMinutesPart());
     }
 
-    /**
-     * @return array
-     */
     public function providerToMinutesPart() : array
     {
         return [
@@ -1333,18 +1192,12 @@ public function providerToMinutesPart() : array
 
     /**
      * @dataProvider providerToSecondsPart
-     *
-     * @param Duration $duration
-     * @param int      $seconds
      */
-    public function testToSecondsPart(Duration $duration, int $seconds)
+    public function testToSecondsPart(Duration $duration, int $seconds): void
     {
         $this->assertSame($seconds, $duration->toSecondsPart());
     }
 
-    /**
-     * @return array
-     */
     public function providerToSecondsPart() : array
     {
         return [
@@ -1359,18 +1212,12 @@ public function providerToSecondsPart() : array
 
     /**
      * @dataProvider providerToMillis
-     *
-     * @param Duration $duration
-     * @param int      $millis
      */
-    public function testToMillis(Duration $duration, int $millis)
+    public function testToMillis(Duration $duration, int $millis): void
     {
         $this->assertSame($millis, $duration->toMillis());
     }
 
-    /**
-     * @return array
-     */
     public function providerToMillis() : array
     {
         return [
@@ -1384,18 +1231,13 @@ public function providerToMillis() : array
 
     /**
      * @dataProvider providerToMillisOutOfRange
-     *
-     * @param Duration $duration
      */
-    public function testToMillisOutOfRange(Duration $duration)
+    public function testToMillisOutOfRange(Duration $duration): void
     {
         $this->expectException(ArithmeticError::class);
         $duration->toMillis();
     }
 
-    /**
-     * @return array
-     */
     public function providerToMillisOutOfRange() : array
     {
         return [
@@ -1406,18 +1248,12 @@ public function providerToMillisOutOfRange() : array
 
     /**
      * @dataProvider providerToMillisPart
-     *
-     * @param Duration $duration
-     * @param int      $millis
      */
-    public function testToMillisPart(Duration $duration, int $millis)
+    public function testToMillisPart(Duration $duration, int $millis): void
     {
         $this->assertSame($millis, $duration->toMillisPart());
     }
 
-    /**
-     * @return array
-     */
     public function providerToMillisPart() : array
     {
         return [
@@ -1432,18 +1268,12 @@ public function providerToMillisPart() : array
 
     /**
      * @dataProvider providerToNanos
-     *
-     * @param Duration $duration
-     * @param int      $nanos
      */
-    public function testToNanos(Duration $duration, int $nanos)
+    public function testToNanos(Duration $duration, int $nanos): void
     {
         $this->assertSame($nanos, $duration->toNanos());
     }
 
-    /**
-     * @return array
-     */
     public function providerToNanos() : array
     {
         return [
@@ -1456,18 +1286,12 @@ public function providerToNanos() : array
 
     /**
      * @dataProvider providerToNanosPart
-     *
-     * @param Duration $duration
-     * @param int      $nanos
      */
-    public function testToNanosPart(Duration $duration, int $nanos)
+    public function testToNanosPart(Duration $duration, int $nanos): void
     {
         $this->assertSame($nanos, $duration->toNanosPart());
     }
 
-    /**
-     * @return array
-     */
     public function providerToNanosPart() : array
     {
         return [
@@ -1482,31 +1306,20 @@ public function providerToNanosPart() : array
 
     /**
      * @dataProvider providerToString
-     *
-     * @param int    $seconds
-     * @param int    $nanos
-     * @param string $expected
      */
-    public function testJsonSerialize(int $seconds, int $nanos, string $expected)
+    public function testJsonSerialize(int $seconds, int $nanos, string $expected): void
     {
         $this->assertSame(json_encode($expected), json_encode(Duration::ofSeconds($seconds, $nanos)));
     }
 
     /**
      * @dataProvider providerToString
-     *
-     * @param int    $seconds
-     * @param int    $nanos
-     * @param string $expected
      */
-    public function testToString(int $seconds, int $nanos, string $expected)
+    public function testToString(int $seconds, int $nanos, string $expected): void
     {
         $this->assertSame($expected, (string) Duration::ofSeconds($seconds, $nanos));
     }
 
-    /**
-     * @return array
-     */
     public function providerToString() : array
     {
         return [
diff --git a/tests/Field/DayOfMonthTest.php b/tests/Field/DayOfMonthTest.php
index 7110607..9b6f66d 100644
--- a/tests/Field/DayOfMonthTest.php
+++ b/tests/Field/DayOfMonthTest.php
@@ -15,7 +15,7 @@ class DayOfMonthTest extends AbstractTestCase
     /**
      * @doesNotPerformAssertions
      */
-    public function testCheckWithNullMonthOfYear()
+    public function testCheckWithNullMonthOfYear(): void
     {
         DayOfMonth::check(31);
     }
diff --git a/tests/Field/WeekOfYearTest.php b/tests/Field/WeekOfYearTest.php
index 293e82b..cfe3f8f 100644
--- a/tests/Field/WeekOfYearTest.php
+++ b/tests/Field/WeekOfYearTest.php
@@ -13,7 +13,7 @@
  */
 class WeekOfYearTest extends AbstractTestCase
 {
-    public function testCheckShouldThrowDateTimeExceptionWithFieldNotInRange()
+    public function testCheckShouldThrowDateTimeExceptionWithFieldNotInRange(): void
     {
         $this->expectException(DateTimeException::class);
         $this->expectExceptionMessage('Invalid week-of-year: -1 is not in the range 1 to 53.');
@@ -21,7 +21,7 @@ public function testCheckShouldThrowDateTimeExceptionWithFieldNotInRange()
         WeekOfYear::check(-1);
     }
 
-    public function testCheckShouldThrowDateTimeExceptionWith52WeekYear()
+    public function testCheckShouldThrowDateTimeExceptionWith52WeekYear(): void
     {
         $this->expectException(DateTimeException::class);
         $this->expectExceptionMessage('Year 2000 does not have 53 weeks');
diff --git a/tests/InstantTest.php b/tests/InstantTest.php
index 1dde255..8418c9c 100644
--- a/tests/InstantTest.php
+++ b/tests/InstantTest.php
@@ -19,11 +19,11 @@ class InstantTest extends AbstractTestCase
      * @dataProvider providerOf
      *
      * @param int $seconds         The duration in seconds.
-     * @param int $nanoAdjustment  The nanoseconds adjustement to the duration.
+     * @param int $nanoAdjustment  The nanoseconds adjustment to the duration.
      * @param int $expectedSeconds The expected adjusted duration seconds.
      * @param int $expectedNanos   The expected adjusted duration nanoseconds.
      */
-    public function testOf(int $seconds, int $nanoAdjustment, int $expectedSeconds, int $expectedNanos)
+    public function testOf(int $seconds, int $nanoAdjustment, int $expectedSeconds, int $expectedNanos): void
     {
         $duration = Instant::of($seconds, $nanoAdjustment);
 
@@ -31,9 +31,6 @@ public function testOf(int $seconds, int $nanoAdjustment, int $expectedSeconds,
         $this->assertSame($expectedNanos, $duration->getNano());
     }
 
-    /**
-     * @return array
-     */
     public function providerOf() : array
     {
         return [
@@ -48,23 +45,23 @@ public function providerOf() : array
         ];
     }
 
-    public function testEpoch()
+    public function testEpoch(): void
     {
         $this->assertInstantIs(0, 0, Instant::epoch());
     }
 
-    public function testNow()
+    public function testNow(): void
     {
         $clock = new FixedClock(Instant::of(123456789, 987654321));
         $this->assertInstantIs(123456789, 987654321, Instant::now($clock));
     }
 
-    public function testMin()
+    public function testMin(): void
     {
         $this->assertInstantIs(~\PHP_INT_MAX, 0, Instant::min());
     }
 
-    public function testMax()
+    public function testMax(): void
     {
         $this->assertInstantIs(\PHP_INT_MAX, 999999999, Instant::max());
     }
@@ -79,7 +76,7 @@ public function testMax()
      * @param int $expectedSecond The expected second of the result.
      * @param int $expectedNano   The expected nano of the result.
      */
-    public function testPlus(int $second, int $nano, int $plusSeconds, int $plusNanos, int $expectedSecond, int $expectedNano)
+    public function testPlus(int $second, int $nano, int $plusSeconds, int $plusNanos, int $expectedSecond, int $expectedNano): void
     {
         $result = Instant::of($second, $nano)->plus(Duration::ofSeconds($plusSeconds, $plusNanos));
         $this->assertInstantIs($expectedSecond, $expectedNano, $result);
@@ -95,15 +92,12 @@ public function testPlus(int $second, int $nano, int $plusSeconds, int $plusNano
      * @param int $expectedSecond The expected second of the result.
      * @param int $expectedNano   The expected nano of the result.
      */
-    public function testMinus(int $second, int $nano, int $plusSeconds, int $plusNanos, int $expectedSecond, int $expectedNano)
+    public function testMinus(int $second, int $nano, int $plusSeconds, int $plusNanos, int $expectedSecond, int $expectedNano): void
     {
         $result = Instant::of($second, $nano)->minus(Duration::ofSeconds(-$plusSeconds, -$plusNanos));
         $this->assertInstantIs($expectedSecond, $expectedNano, $result);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlus() : array
     {
         return [
@@ -123,7 +117,7 @@ public function providerPlus() : array
      * @param int $plusSeconds    The number of seconds to add.
      * @param int $expectedSecond The expected second of the result.
      */
-    public function testPlusSeconds(int $second, int $nano, int $plusSeconds, int $expectedSecond)
+    public function testPlusSeconds(int $second, int $nano, int $plusSeconds, int $expectedSecond): void
     {
         $result = Instant::of($second, $nano)->plusSeconds($plusSeconds);
         $this->assertInstantIs($expectedSecond, $nano, $result);
@@ -137,15 +131,12 @@ public function testPlusSeconds(int $second, int $nano, int $plusSeconds, int $e
      * @param int $plusSeconds    The number of seconds to add.
      * @param int $expectedSecond The expected second of the result.
      */
-    public function testMinusSeconds(int $second, int $nano, int $plusSeconds, int $expectedSecond)
+    public function testMinusSeconds(int $second, int $nano, int $plusSeconds, int $expectedSecond): void
     {
         $result = Instant::of($second, $nano)->minusSeconds(-$plusSeconds);
         $this->assertInstantIs($expectedSecond, $nano, $result);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusSeconds() : array
     {
         return [
@@ -167,7 +158,7 @@ public function providerPlusSeconds() : array
      * @param int $plusMinutes    The number of minutes to add.
      * @param int $expectedSecond The expected second of the result.
      */
-    public function testPlusMinutes(int $second, int $nano, int $plusMinutes, int $expectedSecond)
+    public function testPlusMinutes(int $second, int $nano, int $plusMinutes, int $expectedSecond): void
     {
         $result = Instant::of($second, $nano)->plusMinutes($plusMinutes);
         $this->assertInstantIs($expectedSecond, $nano, $result);
@@ -181,15 +172,12 @@ public function testPlusMinutes(int $second, int $nano, int $plusMinutes, int $e
      * @param int $plusMinutes    The number of minutes to add.
      * @param int $expectedSecond The expected second of the result.
      */
-    public function testMinusMinutes(int $second, int $nano, int $plusMinutes, int $expectedSecond)
+    public function testMinusMinutes(int $second, int $nano, int $plusMinutes, int $expectedSecond): void
     {
         $result = Instant::of($second, $nano)->minusMinutes(-$plusMinutes);
         $this->assertInstantIs($expectedSecond, $nano, $result);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusMinutes() : array
     {
         return [
@@ -225,15 +213,12 @@ public function testPlusHours(int $second, int $nano, int $plusHours, int $expec
      * @param int $plusHours      The number of hours to add.
      * @param int $expectedSecond The expected second of the result.
      */
-    public function testMinusHours(int $second, int $nano, int $plusHours, int $expectedSecond)
+    public function testMinusHours(int $second, int $nano, int $plusHours, int $expectedSecond): void
     {
         $result = Instant::of($second, $nano)->minusHours(-$plusHours);
         $this->assertInstantIs($expectedSecond, $nano, $result);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusHours() : array
     {
         return [
@@ -255,7 +240,7 @@ public function providerPlusHours() : array
      * @param int $plusDays       The number of days to add.
      * @param int $expectedSecond The expected second of the result.
      */
-    public function testPlusDays(int $second, int $nano, int $plusDays, int $expectedSecond)
+    public function testPlusDays(int $second, int $nano, int $plusDays, int $expectedSecond): void
     {
         $result = Instant::of($second, $nano)->plusDays($plusDays);
         $this->assertInstantIs($expectedSecond, $nano, $result);
@@ -269,15 +254,12 @@ public function testPlusDays(int $second, int $nano, int $plusDays, int $expecte
      * @param int $plusDays       The number of days to add.
      * @param int $expectedSecond The expected second of the result.
      */
-    public function testMinusDays(int $second, int $nano, int $plusDays, int $expectedSecond)
+    public function testMinusDays(int $second, int $nano, int $plusDays, int $expectedSecond): void
     {
         $result = Instant::of($second, $nano)->minusDays(-$plusDays);
         $this->assertInstantIs($expectedSecond, $nano, $result);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusDays() : array
     {
         return [
@@ -291,13 +273,13 @@ public function providerPlusDays() : array
         ];
     }
 
-    public function testWithEpochSecond()
+    public function testWithEpochSecond(): void
     {
         $instant = Instant::of(1234567890, 987654321);
         $this->assertInstantIs(2345678901, 987654321, $instant->withEpochSecond(2345678901));
     }
 
-    public function testWithNano()
+    public function testWithNano(): void
     {
         $instant = Instant::of(1234567890, 987654321);
         $this->assertInstantIs(1234567890, 123456789, $instant->withNano(123456789));
@@ -305,10 +287,8 @@ public function testWithNano()
 
     /**
      * @dataProvider providerWithInvalidNanoThrowsException
-     *
-     * @param int $nano
      */
-    public function testWithInvalidNanoThrowsException(int $nano)
+    public function testWithInvalidNanoThrowsException(int $nano): void
     {
         $instant = Instant::of(1234567890, 987654321);
 
@@ -316,9 +296,6 @@ public function testWithInvalidNanoThrowsException(int $nano)
         $instant->withNano($nano);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidNanoThrowsException() : array
     {
         return [
@@ -336,7 +313,7 @@ public function providerWithInvalidNanoThrowsException() : array
      * @param int $n2  The nanosecond adjustment of the 2nd instant.
      * @param int $cmp The expected comparison value.
      */
-    public function testCompareTo(int $s1, int $n1, int $s2, int $n2, int $cmp)
+    public function testCompareTo(int $s1, int $n1, int $s2, int $n2, int $cmp): void
     {
         $this->assertSame($cmp, Instant::of($s1, $n1)->compareTo(Instant::of($s2, $n2)));
     }
@@ -350,7 +327,7 @@ public function testCompareTo(int $s1, int $n1, int $s2, int $n2, int $cmp)
      * @param int $n2  The nanosecond adjustment of the 2nd instant.
      * @param int $cmp The comparison value.
      */
-    public function testIsEqualTo(int $s1, int $n1, int $s2, int $n2, int $cmp)
+    public function testIsEqualTo(int $s1, int $n1, int $s2, int $n2, int $cmp): void
     {
         $this->assertSame($cmp === 0, Instant::of($s1, $n1)->isEqualTo(Instant::of($s2, $n2)));
     }
@@ -364,7 +341,7 @@ public function testIsEqualTo(int $s1, int $n1, int $s2, int $n2, int $cmp)
      * @param int $n2  The nanosecond adjustment of the 2nd instant.
      * @param int $cmp The comparison value.
      */
-    public function testIsAfter(int $s1, int $n1, int $s2, int $n2, int $cmp)
+    public function testIsAfter(int $s1, int $n1, int $s2, int $n2, int $cmp): void
     {
         $this->assertSame($cmp === 1, Instant::of($s1, $n1)->isAfter(Instant::of($s2, $n2)));
     }
@@ -378,7 +355,7 @@ public function testIsAfter(int $s1, int $n1, int $s2, int $n2, int $cmp)
      * @param int $n2  The nanosecond adjustment of the 2nd instant.
      * @param int $cmp The comparison value.
      */
-    public function testIsAfterOrEqualTo(int $s1, int $n1, int $s2, int $n2, int $cmp)
+    public function testIsAfterOrEqualTo(int $s1, int $n1, int $s2, int $n2, int $cmp): void
     {
         $this->assertSame($cmp >= 0, Instant::of($s1, $n1)->isAfterOrEqualTo(Instant::of($s2, $n2)));
     }
@@ -392,7 +369,7 @@ public function testIsAfterOrEqualTo(int $s1, int $n1, int $s2, int $n2, int $cm
      * @param int $n2  The nanosecond adjustment of the 2nd instant.
      * @param int $cmp The comparison value.
      */
-    public function testIsBefore(int $s1, int $n1, int $s2, int $n2, int $cmp)
+    public function testIsBefore(int $s1, int $n1, int $s2, int $n2, int $cmp): void
     {
         $this->assertSame($cmp === -1, Instant::of($s1, $n1)->isBefore(Instant::of($s2, $n2)));
     }
@@ -406,7 +383,7 @@ public function testIsBefore(int $s1, int $n1, int $s2, int $n2, int $cmp)
      * @param int $n2  The nanosecond adjustment of the 2nd instant.
      * @param int $cmp The comparison value.
      */
-    public function testIsBeforeOrEqualTo(int $s1, int $n1, int $s2, int $n2, int $cmp)
+    public function testIsBeforeOrEqualTo(int $s1, int $n1, int $s2, int $n2, int $cmp): void
     {
         $this->assertSame($cmp <= 0, Instant::of($s1, $n1)->isBeforeOrEqualTo(Instant::of($s2, $n2)));
     }
@@ -420,7 +397,7 @@ public function testIsBeforeOrEqualTo(int $s1, int $n1, int $s2, int $n2, int $c
      * @param int $nowNano    The nanosecond adjustment to the current time.
      * @param int $cmp        The comparison value.
      */
-    public function testIsFuture(int $testSecond, int $testNano, int $nowSecond, int $nowNano, int $cmp)
+    public function testIsFuture(int $testSecond, int $testNano, int $nowSecond, int $nowNano, int $cmp): void
     {
         $clock = new FixedClock(Instant::of($nowSecond, $nowNano));
         $this->assertSame($cmp === 1, Instant::of($testSecond, $testNano)->isFuture($clock));
@@ -435,15 +412,12 @@ public function testIsFuture(int $testSecond, int $testNano, int $nowSecond, int
      * @param int $nowNano    The nanosecond adjustment to the current time.
      * @param int $cmp        The comparison value.
      */
-    public function testIsPast(int $testSecond, int $testNano, int $nowSecond, int $nowNano, int $cmp)
+    public function testIsPast(int $testSecond, int $testNano, int $nowSecond, int $nowNano, int $cmp): void
     {
         $clock = new FixedClock(Instant::of($nowSecond, $nowNano));
         $this->assertSame($cmp === -1, Instant::of($testSecond, $testNano)->isPast($clock));
     }
 
-    /**
-     * @return array
-     */
     public function providerCompareTo() : array
     {
         return [
@@ -538,7 +512,7 @@ public function providerCompareTo() : array
      * @param int  $nanos     The nano seconds value.
      * @param bool $isBetween Check the secs and nanos are between.
      */
-    public function testIsBetweenInclusive(int $seconds, int $nanos, $isBetween)
+    public function testIsBetweenInclusive(int $seconds, int $nanos, $isBetween): void
     {
         $this->assertSame($isBetween, Instant::of($seconds, $nanos)->isBetweenInclusive(
             Instant::of(-1, -1),
@@ -553,7 +527,7 @@ public function testIsBetweenInclusive(int $seconds, int $nanos, $isBetween)
      * @param int  $nanos     The nano seconds value.
      * @param bool $isBetween Check the secs and nanos are between.
      */
-    public function testIsBetweenExclusive(int $seconds, int $nanos, $isBetween)
+    public function testIsBetweenExclusive(int $seconds, int $nanos, $isBetween): void
     {
         $this->assertSame($isBetween, Instant::of($seconds, $nanos)->isBetweenExclusive(
             Instant::of(-1, -1),
@@ -561,9 +535,6 @@ public function testIsBetweenExclusive(int $seconds, int $nanos, $isBetween)
         ));
     }
 
-    /**
-     * @return array
-     */
     public function providerIsBetweenExclusive() : array
     {
         return [
@@ -608,14 +579,11 @@ public function providerIsBetweenInclusive() : array
      * @param int    $nano     The nano adjustment.
      * @param string $expected The expected decimal output.
      */
-    public function testToDecimal(int $second, int $nano, string $expected)
+    public function testToDecimal(int $second, int $nano, string $expected): void
     {
         $this->assertSame($expected, Instant::of($second, $nano)->toDecimal());
     }
 
-    /**
-     * @return array
-     */
     public function providerToDecimal()
     {
         return [
@@ -640,7 +608,7 @@ public function providerToDecimal()
      * @param int    $nano           The nano adjustment to the epoch second.
      * @param string $expectedString The expected string output.
      */
-    public function testJsonSerialize(int $epochSecond, int $nano, string $expectedString)
+    public function testJsonSerialize(int $epochSecond, int $nano, string $expectedString): void
     {
         $this->assertSame(json_encode($expectedString), json_encode(Instant::of($epochSecond, $nano)));
     }
@@ -652,14 +620,11 @@ public function testJsonSerialize(int $epochSecond, int $nano, string $expectedS
      * @param int    $nano           The nano adjustment to the epoch second.
      * @param string $expectedString The expected string output.
      */
-    public function testToString(int $epochSecond, int $nano, string $expectedString)
+    public function testToString(int $epochSecond, int $nano, string $expectedString): void
     {
         $this->assertSame($expectedString, (string) Instant::of($epochSecond, $nano));
     }
 
-    /**
-     * @return array
-     */
     public function providerToString() : array
     {
         return [
@@ -674,7 +639,7 @@ public function providerToString() : array
         ];
     }
 
-    public function testAtTimeZone()
+    public function testAtTimeZone(): void
     {
         $timeZone = TimeZone::utc();
         $instant = Instant::of(1000000000);
diff --git a/tests/IntervalTest.php b/tests/IntervalTest.php
index e57e218..25ed868 100644
--- a/tests/IntervalTest.php
+++ b/tests/IntervalTest.php
@@ -13,7 +13,7 @@
  */
 class IntervalTest extends AbstractTestCase
 {
-    public function testEndInstantIsNotBeforeStartInstant()
+    public function testEndInstantIsNotBeforeStartInstant(): void
     {
         $start = Instant::of(2000000000, 987654321);
         $end = Instant::of(2000000009, 123456789);
@@ -24,7 +24,7 @@ public function testEndInstantIsNotBeforeStartInstant()
         new Interval($end, $start);
     }
 
-    public function testGetStartEnd()
+    public function testGetStartEnd(): void
     {
         $start = Instant::of(2000000000, 987654321);
         $end = Instant::of(2000000009, 123456789);
@@ -38,7 +38,7 @@ public function testGetStartEnd()
     /**
      * @depends testGetStartEnd
      */
-    public function testWithStart()
+    public function testWithStart(): void
     {
         $interval = new Interval(
             Instant::of(2000000000),
@@ -61,7 +61,7 @@ public function testWithStart()
     /**
      * @depends testGetStartEnd
      */
-    public function testWithEnd()
+    public function testWithEnd(): void
     {
         $interval = new Interval(
             Instant::of(2000000000),
@@ -81,7 +81,7 @@ public function testWithEnd()
         $this->assertInstantIs(2000000002, 222222222, $newInterval->getEnd());
     }
 
-    public function testGetDuration()
+    public function testGetDuration(): void
     {
         $interval = new Interval(
             Instant::of(1999999999, 555555),
@@ -93,7 +93,7 @@ public function testGetDuration()
         $this->assertDurationIs(1, 999444556, $duration);
     }
 
-    public function testJsonSerialize()
+    public function testJsonSerialize(): void
     {
         $interval = new Interval(
             Instant::of(1000000000),
@@ -103,7 +103,7 @@ public function testJsonSerialize()
         $this->assertSame(json_encode('2001-09-09T01:46:40Z/2033-05-18T03:33:20Z'), json_encode($interval));
     }
 
-    public function testToString()
+    public function testToString(): void
     {
         $interval = new Interval(
             Instant::of(1000000000),
diff --git a/tests/LocalDateRangeTest.php b/tests/LocalDateRangeTest.php
index bf61aec..01f93e7 100644
--- a/tests/LocalDateRangeTest.php
+++ b/tests/LocalDateRangeTest.php
@@ -14,7 +14,7 @@
  */
 class LocalDateRangeTest extends AbstractTestCase
 {
-    public function testOf()
+    public function testOf(): void
     {
         $this->assertLocalDateRangeIs(2001, 2, 3, 2004, 5, 6, LocalDateRange::of(
             LocalDate::of(2001, 2, 3),
@@ -22,7 +22,7 @@ public function testOf()
         ));
     }
 
-    public function testOfInvalidRangeThrowsException()
+    public function testOfInvalidRangeThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
 
@@ -43,14 +43,11 @@ public function testOfInvalidRangeThrowsException()
      * @param int    $m2   The expected end month.
      * @param int    $d2   The expected end day.
      */
-    public function testParse(string $text, int $y1, int $m1, int $d1, int $y2, int $m2, int $d2)
+    public function testParse(string $text, int $y1, int $m1, int $d1, int $y2, int $m2, int $d2): void
     {
         $this->assertLocalDateRangeIs($y1, $m1, $d1, $y2, $m2, $d2, LocalDateRange::parse($text));
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -65,15 +62,12 @@ public function providerParse() : array
      *
      * @param string $text The invalid text to parse.
      */
-    public function testParseInvalidRangeThrowsException(string $text)
+    public function testParseInvalidRangeThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         LocalDateRange::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidRangeThrowsException() : array
     {
         return [
@@ -96,7 +90,7 @@ public function providerParseInvalidRangeThrowsException() : array
      * @param string $testRange The string representation of the range to test.
      * @param bool   $isEqual   Whether this range is expected to be equal to our range.
      */
-    public function testIsEqualTo(string $testRange, bool $isEqual)
+    public function testIsEqualTo(string $testRange, bool $isEqual): void
     {
         $this->assertSame($isEqual, LocalDateRange::of(
             LocalDate::of(2001, 2, 3),
@@ -104,9 +98,6 @@ public function testIsEqualTo(string $testRange, bool $isEqual)
         )->isEqualTo(LocalDateRange::parse($testRange)));
     }
 
-    /**
-     * @return array
-     */
     public function providerIsEqualTo() : array
     {
         return [
@@ -122,19 +113,12 @@ public function providerIsEqualTo() : array
 
     /**
      * @dataProvider providerContains
-     *
-     * @param string $range
-     * @param string $date
-     * @param bool   $contains
      */
-    public function testContains(string $range, string $date, bool $contains)
+    public function testContains(string $range, string $date, bool $contains): void
     {
         $this->assertSame($contains, LocalDateRange::parse($range)->contains(LocalDate::parse($date)));
     }
 
-    /**
-     * @return array
-     */
     public function providerContains() : array
     {
         return [
@@ -150,7 +134,7 @@ public function providerContains() : array
         ];
     }
 
-    public function testIterator()
+    public function testIterator(): void
     {
         $start = LocalDate::of(2013, 12, 29);
         $end   = LocalDate::of(2014, 1, 3);
@@ -183,14 +167,11 @@ public function testIterator()
      * @param string $range The date range string representation.
      * @param int    $count The expected day count.
      */
-    public function testCount(string $range, int $count)
+    public function testCount(string $range, int $count): void
     {
         $this->assertCount($count, LocalDateRange::parse($range));
     }
 
-    /**
-     * @return array
-     */
     public function providerCount() : array
     {
         return [
@@ -203,7 +184,7 @@ public function providerCount() : array
         ];
     }
 
-    public function testJsonSerialize()
+    public function testJsonSerialize(): void
     {
         $this->assertSame(json_encode('2008-12-31/2011-01-01'), json_encode(LocalDateRange::of(
             LocalDate::of(2008, 12, 31),
@@ -211,7 +192,7 @@ public function testJsonSerialize()
         )));
     }
 
-    public function testToString()
+    public function testToString(): void
     {
         $this->assertSame('2008-12-31/2011-01-01', (string) LocalDateRange::of(
             LocalDate::of(2008, 12, 31),
@@ -222,11 +203,11 @@ public function testToString()
     /**
      * @dataProvider providerToDatePeriod
      *
-     * @param string $range The date-time string that will be parse()d by LocalDateRange.
+     * @param string $range         The date-time string that will be parse()d by LocalDateRange.
      * @param string $expectedStart The expected output from the native DateTime object.
-     * @param string $expectedEnd The expected output from the native DateTime object.
+     * @param string $expectedEnd   The expected output from the native DateTime object.
      */
-    public function testToDatePeriod(string $range, string $expectedStart, string $expectedEnd)
+    public function testToDatePeriod(string $range, string $expectedStart, string $expectedEnd): void
     {
         $range = LocalDateRange::parse($range);
 
@@ -235,6 +216,7 @@ public function testToDatePeriod(string $range, string $expectedStart, string $e
         $rangeArray = iterator_to_array($range);
         $periodArray = iterator_to_array($period);
         $zip = array_map(null, $rangeArray, $periodArray);
+
         foreach ($zip as [$date, $dateTime]) {
             $this->assertTrue($date->isEqualTo(LocalDate::fromDateTime($dateTime)));
         }
@@ -244,9 +226,6 @@ public function testToDatePeriod(string $range, string $expectedStart, string $e
         $this->assertSame($expectedEnd, $period->end->format('Y-m-d\TH:i:s.uO'));
     }
 
-    /**
-     * @return array
-     */
     public function providerToDatePeriod(): array
     {
         return [
@@ -258,12 +237,8 @@ public function providerToDatePeriod(): array
 
     /**
      * @dataProvider providerIntersectsWith
-     *
-     * @param string $a
-     * @param string $b
-     * @param bool $expectedResult
      */
-    public function testIntersectsWith(string $a, string $b, bool $expectedResult)
+    public function testIntersectsWith(string $a, string $b, bool $expectedResult): void
     {
         $aRange = LocalDateRange::parse($a);
         $bRange = LocalDateRange::parse($b);
@@ -289,12 +264,8 @@ public function providerIntersectsWith() : array
 
     /**
      * @dataProvider providerGetIntersectionWith
-     *
-     * @param string $a
-     * @param string $b
-     * @param string $expectedIntersection
      */
-    public function testGetIntersectionWith(string $a, string $b, string $expectedIntersection)
+    public function testGetIntersectionWith(string $a, string $b, string $expectedIntersection): void
     {
         $aRange = LocalDateRange::parse($a);
         $bRange = LocalDateRange::parse($b);
@@ -315,7 +286,7 @@ public function providerGetIntersectionWith() : array
         ];
     }
 
-    public function testGetIntersectionInvalidParams()
+    public function testGetIntersectionInvalidParams(): void
     {
         $aRange = LocalDateRange::parse('2010-01-01/2010-03-01');
         $bRange = LocalDateRange::parse('2033-01-02/2033-01-02');
diff --git a/tests/LocalDateTest.php b/tests/LocalDateTest.php
index fd5c401..73e5040 100644
--- a/tests/LocalDateTest.php
+++ b/tests/LocalDateTest.php
@@ -18,7 +18,7 @@
  */
 class LocalDateTest extends AbstractTestCase
 {
-    public function testOf()
+    public function testOf(): void
     {
         $this->assertLocalDateIs(2007, 7, 15, LocalDate::of(2007, 7, 15));
     }
@@ -30,15 +30,12 @@ public function testOf()
      * @param int $month The month of the invalid date.
      * @param int $day   The day of the invalid date.
      */
-    public function testOfInvalidDateThrowsException(int $year, int $month, int $day)
+    public function testOfInvalidDateThrowsException(int $year, int $month, int $day): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of($year, $month, $day);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInvalidDateThrowsException() : array
     {
         return [
@@ -59,15 +56,12 @@ public function providerOfInvalidDateThrowsException() : array
      * @param int $year      The year.
      * @param int $dayOfYear The day-of-year.
      */
-    public function testOfInvalidYearDayThrowsException(int $year, int $dayOfYear)
+    public function testOfInvalidYearDayThrowsException(int $year, int $dayOfYear): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::ofYearDay($year, $dayOfYear);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInvalidYearDayThrowsException() : array
     {
         return [
@@ -87,12 +81,12 @@ public function providerOfInvalidYearDayThrowsException() : array
      * @param int $month    The expected month.
      * @param int $day      The expected day.
      */
-    public function testOfEpochDay(int $epochDay, int $year, int $month, int $day)
+    public function testOfEpochDay(int $epochDay, int $year, int $month, int $day): void
     {
         $this->assertLocalDateIs($year, $month, $day, LocalDate::ofEpochDay($epochDay));
     }
 
-    public function testOfEpochDayOutOfRangeThrowsException()
+    public function testOfEpochDayOutOfRangeThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::ofEpochDay(500000000);
@@ -106,14 +100,11 @@ public function testOfEpochDayOutOfRangeThrowsException()
      * @param int $month    The month.
      * @param int $day      The day.
      */
-    public function testToEpochDay(int $epochDay, int $year, int $month, int $day)
+    public function testToEpochDay(int $epochDay, int $year, int $month, int $day): void
     {
         $this->assertSame($epochDay, LocalDate::of($year, $month, $day)->toEpochDay());
     }
 
-    /**
-     * @return array
-     */
     public function providerEpochDay() : array
     {
         return [
@@ -135,7 +126,7 @@ public function providerEpochDay() : array
         ];
     }
 
-    public function testFromDateTime()
+    public function testFromDateTime(): void
     {
         $dateTime = new \DateTime('2018-07-21');
         $this->assertLocalDateIs(2018, 7, 21, LocalDate::fromDateTime($dateTime));
@@ -150,15 +141,12 @@ public function testFromDateTime()
      * @param int    $month       The expected month.
      * @param int    $day         The expected day.
      */
-    public function testNow(int $epochSecond, string $timeZone, int $year, int $month, int $day)
+    public function testNow(int $epochSecond, string $timeZone, int $year, int $month, int $day): void
     {
         $clock = new FixedClock(Instant::of($epochSecond));
         $this->assertLocalDateIs($year, $month, $day, LocalDate::now(TimeZone::parse($timeZone), $clock));
     }
 
-    /**
-     * @return array
-     */
     public function providerNow() : array
     {
         return [
@@ -169,31 +157,24 @@ public function providerNow() : array
         ];
     }
 
-    public function testMin()
+    public function testMin(): void
     {
         $this->assertLocalDateIs(Year::MIN_VALUE, 1, 1, LocalDate::min());
     }
 
-    public function testMax()
+    public function testMax(): void
     {
         $this->assertLocalDateIs(Year::MAX_VALUE, 12, 31, LocalDate::max());
     }
 
     /**
      * @dataProvider providerGetYearMonth
-     *
-     * @param int $year
-     * @param int $month
-     * @param int $day
      */
-    public function testGetYearMonth(int $year, int $month, int $day)
+    public function testGetYearMonth(int $year, int $month, int $day): void
     {
         $this->assertYearMonthIs($year, $month, LocalDate::of($year, $month, $day)->getYearMonth());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetYearMonth() : array
     {
         return [
@@ -211,14 +192,11 @@ public function providerGetYearMonth() : array
      * @param int $day       The day-of-month to test.
      * @param int $dayOfWeek The expected day-of-week number.
      */
-    public function testGetDayOfWeek(int $year, int $month, int $day, int $dayOfWeek)
+    public function testGetDayOfWeek(int $year, int $month, int $day, int $dayOfWeek): void
     {
         $this->assertDayOfWeekIs($dayOfWeek, LocalDate::of($year, $month, $day)->getDayOfWeek());
     }
 
-    /**
-     * @return array
-     */
     public function providerDayOfWeek() : array
     {
         return [
@@ -257,7 +235,7 @@ public function providerDayOfWeek() : array
      * @param int $day       The expected day.
      * @param int $dayOfYear The day-of-year.
      */
-    public function testOfYearDay(int $year, int $month, int $day, int $dayOfYear)
+    public function testOfYearDay(int $year, int $month, int $day, int $dayOfYear): void
     {
         $this->assertLocalDateIs($year, $month, $day, LocalDate::ofYearDay($year, $dayOfYear));
     }
@@ -270,14 +248,11 @@ public function testOfYearDay(int $year, int $month, int $day, int $dayOfYear)
      * @param int $day       The day-of-month to test.
      * @param int $dayOfYear The expected day-of-year number.
      */
-    public function testGetDayOfYear(int $year, int $month, int $day, int $dayOfYear)
+    public function testGetDayOfYear(int $year, int $month, int $day, int $dayOfYear): void
     {
         $this->assertSame($dayOfYear, LocalDate::of($year, $month, $day)->getDayOfYear());
     }
 
-    /**
-     * @return array
-     */
     public function providerDayOfYear() : array
     {
         return [
@@ -332,9 +307,6 @@ public function providerDayOfYear() : array
         ];
     }
 
-    /**
-     * @return array
-     */
     public function providerGetYearWeek() : array
     {
         return [
@@ -479,14 +451,8 @@ public function providerGetYearWeek() : array
 
     /**
      * @dataProvider providerGetYearWeek
-     *
-     * @param int $year
-     * @param int $month
-     * @param int $day
-     * @param int $expectedYear
-     * @param int $expectedWeek
      */
-    public function testGetYearWeek(int $year, int $month, int $day, int $expectedYear, int $expectedWeek)
+    public function testGetYearWeek(int $year, int $month, int $day, int $expectedYear, int $expectedWeek): void
     {
         $yearWeek = LocalDate::of($year, $month, $day)->getYearWeek();
         $this->assertYearWeekIs($expectedYear, $expectedWeek, $yearWeek);
@@ -501,15 +467,12 @@ public function testGetYearWeek(int $year, int $month, int $day, int $expectedYe
      * @param int $newYear     The new year.
      * @param int $expectedDay The expected day-of-month of the resulting date.
      */
-    public function testWithYear(int $year, int $month, int $day, int $newYear, int $expectedDay)
+    public function testWithYear(int $year, int $month, int $day, int $newYear, int $expectedDay): void
     {
         $localDate = LocalDate::of($year, $month, $day)->withYear($newYear);
         $this->assertLocalDateIs($newYear, $month, $expectedDay, $localDate);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithYear() : array
     {
         return [
@@ -527,15 +490,12 @@ public function providerWithYear() : array
      *
      * @param int $invalidYear The year to test.
      */
-    public function testWithInvalidYearThrowsException(int $invalidYear)
+    public function testWithInvalidYearThrowsException(int $invalidYear): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of(2001, 2, 3)->withYear($invalidYear);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidYearThrowsException() : array
     {
         return [
@@ -553,15 +513,12 @@ public function providerWithInvalidYearThrowsException() : array
      * @param int $newMonth    The new month.
      * @param int $expectedDay The expected day-of-month of the resulting date.
      */
-    public function testWithMonth(int $year, int $month, int $day, int $newMonth, int $expectedDay)
+    public function testWithMonth(int $year, int $month, int $day, int $newMonth, int $expectedDay): void
     {
         $localDate = LocalDate::of($year, $month, $day)->withMonth($newMonth);
         $this->assertLocalDateIs($year, $newMonth, $expectedDay, $localDate);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithMonth() : array
     {
         return [
@@ -587,15 +544,12 @@ public function providerWithMonth() : array
      *
      * @param int $invalidMonth The month to test.
      */
-    public function testWithInvalidMonthThrowsException(int $invalidMonth)
+    public function testWithInvalidMonthThrowsException(int $invalidMonth): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6))->withMonth($invalidMonth);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidMonthThrowsException() : array
     {
         return [
@@ -612,15 +566,12 @@ public function providerWithInvalidMonthThrowsException() : array
      * @param int $day    The base day-of-month.
      * @param int $newDay The new day-of-month.
      */
-    public function testWithDay(int $year, int $month, int $day, int $newDay)
+    public function testWithDay(int $year, int $month, int $day, int $newDay): void
     {
         $localDate = LocalDate::of($year, $month, $day)->withDay($newDay);
         $this->assertLocalDateIs($year, $month, $newDay, $localDate);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithDay() : array
     {
         return [
@@ -639,15 +590,12 @@ public function providerWithDay() : array
      * @param int $day    The base day-of-month.
      * @param int $newDay The new day-of-month.
      */
-    public function testWithInvalidDayThrowsException(int $year, int $month, int $day, int $newDay)
+    public function testWithInvalidDayThrowsException(int $year, int $month, int $day, int $newDay): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of($year, $month, $day)->withDay($newDay);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidDayThrowsException() : array
     {
         return [
@@ -672,7 +620,7 @@ public function providerWithInvalidDayThrowsException() : array
      * @param int $em The expected month of the result date.
      * @param int $ed The expected day of the result date.
      */
-    public function testPlusPeriod(int $y, int $m, int $d, int $py, int $pm, int $pd, int $ey, int $em, int $ed)
+    public function testPlusPeriod(int $y, int $m, int $d, int $py, int $pm, int $pd, int $ey, int $em, int $ed): void
     {
         $date = LocalDate::of($y, $m, $d);
         $period = Period::of($py, $pm, $pd);
@@ -693,7 +641,7 @@ public function testPlusPeriod(int $y, int $m, int $d, int $py, int $pm, int $pd
      * @param int $em The expected month of the result date.
      * @param int $ed The expected day of the result date.
      */
-    public function testMinusPeriod(int $y, int $m, int $d, int $py, int $pm, int $pd, int $ey, int $em, int $ed)
+    public function testMinusPeriod(int $y, int $m, int $d, int $py, int $pm, int $pd, int $ey, int $em, int $ed): void
     {
         $date = LocalDate::of($y, $m, $d);
         $period = Period::of(-$py, -$pm, -$pd);
@@ -701,9 +649,6 @@ public function testMinusPeriod(int $y, int $m, int $d, int $py, int $pm, int $p
         $this->assertLocalDateIs($ey, $em, $ed, $date->minusPeriod($period));
     }
 
-    /**
-     * @return array
-     */
     public function providerPeriod() : array
     {
         return [
@@ -732,14 +677,11 @@ public function providerPeriod() : array
      * @param int $em The expected resulting month.
      * @param int $ed The expected resulting day.
      */
-    public function testPlusYears(int $y, int $m, int $d, int $ay, int $ey, int $em, int $ed)
+    public function testPlusYears(int $y, int $m, int $d, int $ay, int $ey, int $em, int $ed): void
     {
         $this->assertLocalDateIs($ey, $em, $ed, LocalDate::of($y, $m, $d)->plusYears($ay));
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusYears() : array
     {
         return [
@@ -762,14 +704,11 @@ public function providerPlusYears() : array
      * @param int $em The expected resulting month.
      * @param int $ed The expected resulting day.
      */
-    public function testPlusMonths(int $y, int $m, int $d, int $am, int $ey, int $em, int $ed)
+    public function testPlusMonths(int $y, int $m, int $d, int $am, int $ey, int $em, int $ed): void
     {
         $this->assertLocalDateIs($ey, $em, $ed, LocalDate::of($y, $m, $d)->plusMonths($am));
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusMonths() : array
     {
         return [
@@ -801,14 +740,11 @@ public function providerPlusMonths() : array
      * @param int $em The expected resulting month.
      * @param int $ed The expected resulting day.
      */
-    public function testPlusWeeks(int $y, int $m, int $d, int $aw, int $ey, int $em, int $ed)
+    public function testPlusWeeks(int $y, int $m, int $d, int $aw, int $ey, int $em, int $ed): void
     {
         $this->assertLocalDateIs($ey, $em, $ed, LocalDate::of($y, $m, $d)->plusWeeks($aw));
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusWeeks() : array
     {
         return [
@@ -833,14 +769,11 @@ public function providerPlusWeeks() : array
      * @param int $em The expected resulting month.
      * @param int $ed The expected resulting day.
      */
-    public function testPlusDays(int $y, int $m, int $d, int $ad, int $ey, int $em, int $ed)
+    public function testPlusDays(int $y, int $m, int $d, int $ad, int $ey, int $em, int $ed): void
     {
         $this->assertLocalDateIs($ey, $em, $ed, LocalDate::of($y, $m, $d)->plusDays($ad));
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusDays() : array
     {
         return [
@@ -866,14 +799,11 @@ public function providerPlusDays() : array
      * @param int $em The expected resulting month.
      * @param int $ed The expected resulting day.
      */
-    public function tesMinusYears(int $y, int $m, int $d, int $sy, int $ey, int $em, int $ed)
+    public function tesMinusYears(int $y, int $m, int $d, int $sy, int $ey, int $em, int $ed): void
     {
         $this->assertLocalDateIs($ey, $em, $ed, LocalDate::of($y, $m, $d)->minusYears($sy));
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusYears() : array
     {
         return [
@@ -896,14 +826,11 @@ public function providerMinusYears() : array
      * @param int $em The expected resulting month.
      * @param int $ed The expected resulting day.
      */
-    public function testMinusMonths(int $y, int $m, int $d, int $sm, int $ey, int $em, int $ed)
+    public function testMinusMonths(int $y, int $m, int $d, int $sm, int $ey, int $em, int $ed): void
     {
         $this->assertLocalDateIs($ey, $em, $ed, LocalDate::of($y, $m, $d)->minusMonths($sm));
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusMonths() : array
     {
         return [
@@ -935,14 +862,11 @@ public function providerMinusMonths() : array
      * @param int $em The expected resulting month.
      * @param int $ed The expected resulting day.
      */
-    public function testMinusWeeks(int $y, int $m, int $d, int $sw, int $ey, int $em, int $ed)
+    public function testMinusWeeks(int $y, int $m, int $d, int $sw, int $ey, int $em, int $ed): void
     {
         $this->assertLocalDateIs($ey, $em, $ed, LocalDate::of($y, $m, $d)->minusWeeks($sw));
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusWeeks() : array
     {
         return [
@@ -967,14 +891,11 @@ public function providerMinusWeeks() : array
      * @param int $em The expected resulting month.
      * @param int $ed The expected resulting day.
      */
-    public function testMinusDays(int $y, int $m, int $d, int $sd, int $ey, int $em, int $ed)
+    public function testMinusDays(int $y, int $m, int $d, int $sd, int $ey, int $em, int $ed): void
     {
         $this->assertLocalDateIs($ey, $em, $ed, LocalDate::of($y, $m, $d)->minusDays($sd));
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusDays() : array
     {
         return [
@@ -1002,7 +923,7 @@ public function providerMinusDays() : array
      * @param int $m  The expected number of months in the period.
      * @param int $d  The expected number of days in the period.
      */
-    public function testUntil(int $y1, int $m1, int $d1, int $y2, int $m2, int $d2, int $y, int $m, int $d)
+    public function testUntil(int $y1, int $m1, int $d1, int $y2, int $m2, int $d2, int $y, int $m, int $d): void
     {
         $date1 = LocalDate::of($y1, $m1, $d1);
         $date2 = LocalDate::of($y2, $m2, $d2);
@@ -1010,9 +931,6 @@ public function testUntil(int $y1, int $m1, int $d1, int $y2, int $m2, int $d2,
         $this->assertPeriodIs($y, $m, $d, $date1->until($date2));
     }
 
-    /**
-     * @return array
-     */
     public function providerUntil() : array
     {
         return [
@@ -1111,12 +1029,8 @@ public function providerUntil() : array
 
     /**
      * @dataProvider providerDaysUntil
-     *
-     * @param string $date1
-     * @param string $date2
-     * @param int    $expectedDays
      */
-    public function testDaysUntil(string $date1, string $date2, int $expectedDays)
+    public function testDaysUntil(string $date1, string $date2, int $expectedDays): void
     {
         $date1 = LocalDate::parse($date1);
         $date2 = LocalDate::parse($date2);
@@ -1124,9 +1038,6 @@ public function testDaysUntil(string $date1, string $date2, int $expectedDays)
         $this->assertSame($expectedDays, $date1->daysUntil($date2));
     }
 
-    /**
-     * @return array
-     */
     public function providerDaysUntil() : array
     {
         return [
@@ -1138,7 +1049,7 @@ public function providerDaysUntil() : array
         ];
     }
 
-    public function testAtTime()
+    public function testAtTime(): void
     {
         $localDateTime = LocalDate::of(1, 2, 3)->atTime(LocalTime::of(4, 5, 6, 7));
         $this->assertLocalDateTimeIs(1, 2, 3, 4, 5, 6, 7, $localDateTime);
@@ -1152,7 +1063,7 @@ public function testAtTime()
      * @param int  $d      The day of the date to test (should not matter).
      * @param bool $isLeap Whether the year is a leap year.
      */
-    public function testIsLeapYear(int $y, int $m, int $d, bool $isLeap)
+    public function testIsLeapYear(int $y, int $m, int $d, bool $isLeap): void
     {
         $this->assertSame($isLeap, LocalDate::of($y, $m, $d)->isLeapYear());
     }
@@ -1165,14 +1076,11 @@ public function testIsLeapYear(int $y, int $m, int $d, bool $isLeap)
      * @param int  $d      The day of the date to test (should not matter).
      * @param bool $isLeap Whether the year is a leap year.
      */
-    public function testGetLengthOfYear(int $y, int $m, int $d, bool $isLeap)
+    public function testGetLengthOfYear(int $y, int $m, int $d, bool $isLeap): void
     {
         $this->assertSame($isLeap ? 366 : 365, LocalDate::of($y, $m, $d)->getLengthOfYear());
     }
 
-    /**
-     * @return array
-     */
     public function providerIsLeapYear() : array
     {
         return [
@@ -1196,14 +1104,11 @@ public function providerIsLeapYear() : array
      * @param int $d      The day of the date to test (should not matter).
      * @param int $length The length of the month.
      */
-    public function testGetLengthOfMonth(int $y, int $m, int $d, int $length)
+    public function testGetLengthOfMonth(int $y, int $m, int $d, int $length): void
     {
         $this->assertSame($length, LocalDate::of($y, $m, $d)->getLengthOfMonth());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetLengthOfMonth() : array
     {
         return [
@@ -1230,7 +1135,7 @@ public function providerGetLengthOfMonth() : array
      * @param string $date2 The second date.
      * @param int    $cmp   The comparison value.
      */
-    public function testCompareTo(string $date1, string $date2, int $cmp)
+    public function testCompareTo(string $date1, string $date2, int $cmp): void
     {
         $date1 = LocalDate::parse($date1);
         $date2 = LocalDate::parse($date2);
@@ -1243,9 +1148,6 @@ public function testCompareTo(string $date1, string $date2, int $cmp)
         $this->assertSame($cmp >= 0, $date1->isAfterOrEqualTo($date2));
     }
 
-    /**
-     * @return array
-     */
     public function providerCompareTo() : array
     {
         return [
@@ -1269,7 +1171,7 @@ public function providerCompareTo() : array
      * @param int    $day      The day-of-month.
      * @param string $expected The expected result string.
      */
-    public function testJsonSerialize(int $year, int $month, int $day, string $expected)
+    public function testJsonSerialize(int $year, int $month, int $day, string $expected): void
     {
         $this->assertSame(json_encode($expected), json_encode(LocalDate::of($year, $month, $day)));
     }
@@ -1282,14 +1184,11 @@ public function testJsonSerialize(int $year, int $month, int $day, string $expec
      * @param int    $day      The day-of-month.
      * @param string $expected The expected result string.
      */
-    public function testToString(int $year, int $month, int $day, string $expected)
+    public function testToString(int $year, int $month, int $day, string $expected): void
     {
         $this->assertSame($expected, (string) LocalDate::of($year, $month, $day));
     }
 
-    /**
-     * @return array
-     */
     public function providerToString() : array
     {
         return [
@@ -1298,7 +1197,7 @@ public function providerToString() : array
         ];
     }
 
-    public function testMinMaxOf()
+    public function testMinMaxOf(): void
     {
         $a = LocalDate::of(2015, 9, 30);
         $b = LocalDate::of(2016, 7, 31);
@@ -1308,13 +1207,13 @@ public function testMinMaxOf()
         $this->assertSame($c, LocalDate::maxOf($a, $b, $c));
     }
 
-    public function testMinOfZeroElementsThrowsException()
+    public function testMinOfZeroElementsThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::minOf();
     }
 
-    public function testMaxOfZeroElementsThrowsException()
+    public function testMaxOfZeroElementsThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::maxOf();
@@ -1326,7 +1225,7 @@ public function testMaxOfZeroElementsThrowsException()
      * @param string $dateTime The date-time string that will be parse()d by LocalDate.
      * @param string $expected The expected output from the native DateTime object.
      */
-    public function testToDateTime(string $dateTime, string $expected)
+    public function testToDateTime(string $dateTime, string $expected): void
     {
         $zonedDateTime = LocalDate::parse($dateTime);
         $dateTime = $zonedDateTime->toDateTime();
@@ -1341,7 +1240,7 @@ public function testToDateTime(string $dateTime, string $expected)
      * @param string $dateTime The date-time string that will be parse()d by LocalDate.
      * @param string $expected The expected output from the native DateTime object.
      */
-    public function testToDateTimeImmutable(string $dateTime, string $expected)
+    public function testToDateTimeImmutable(string $dateTime, string $expected): void
     {
         $zonedDateTime = LocalDate::parse($dateTime);
         $dateTime = $zonedDateTime->toDateTimeImmutable();
@@ -1350,10 +1249,7 @@ public function testToDateTimeImmutable(string $dateTime, string $expected)
         $this->assertSame($expected, $dateTime->format('Y-m-d\TH:i:s.uO'));
     }
 
-    /**
-     * @return array
-     */
-    public function providerToDateTime()
+    public function providerToDateTime(): array
     {
         return [
             ['2011-07-31', '2011-07-31T00:00:00.000000+0000'],
diff --git a/tests/LocalDateTimeTest.php b/tests/LocalDateTimeTest.php
index 007c508..cb9caef 100644
--- a/tests/LocalDateTimeTest.php
+++ b/tests/LocalDateTimeTest.php
@@ -16,14 +16,13 @@
 use Brick\DateTime\Period;
 use Brick\DateTime\TimeZone;
 use Brick\DateTime\TimeZoneOffset;
-use Brick\DateTime\ZonedDateTime;
 
 /**
  * Unit tests for class LocalDateTime.
  */
 class LocalDateTimeTest extends AbstractTestCase
 {
-    public function testOf()
+    public function testOf(): void
     {
         $date = LocalDate::of(2001, 12, 23);
         $time = LocalTime::of(12, 34, 56, 987654321);
@@ -31,7 +30,7 @@ public function testOf()
         $this->assertLocalDateTimeIs(2001, 12, 23, 12, 34, 56, 987654321, new LocalDateTime($date, $time));
     }
 
-    public function testFromDateTime()
+    public function testFromDateTime(): void
     {
         $dateTime = new \DateTime('2018-07-21 14:09:10.23456');
         $this->assertLocalDateTimeIs(2018, 7, 21, 14, 9, 10, 234560000, LocalDateTime::fromDateTime($dateTime));
@@ -51,16 +50,13 @@ public function testFromDateTime()
      * @param int $s      The expected second.
      * @param int $n      The expected nano.
      */
-    public function testNow(int $second, int $nano, int $offset, int $y, int $m, int $d, int $h, int $i, int $s, int $n)
+    public function testNow(int $second, int $nano, int $offset, int $y, int $m, int $d, int $h, int $i, int $s, int $n): void
     {
         $clock = new FixedClock(Instant::of($second, $nano));
         $timeZone = TimeZoneOffset::ofTotalSeconds($offset);
         $this->assertLocalDateTimeIs($y, $m, $d, $h, $i, $s, $n, LocalDateTime::now($timeZone, $clock));
     }
 
-    /**
-     * @return array
-     */
     public function providerNow() : array
     {
         return [
@@ -83,14 +79,11 @@ public function providerNow() : array
      * @param int    $s The expected second.
      * @param int    $n The expected nano-of-second.
      */
-    public function testParse(string $t, int $y, int $m, int $d, int $h, int $i, int $s, int $n)
+    public function testParse(string $t, int $y, int $m, int $d, int $h, int $i, int $s, int $n): void
     {
         $this->assertLocalDateTimeIs($y, $m, $d, $h, $i, $s, $n, LocalDateTime::parse($t));
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -103,18 +96,13 @@ public function providerParse() : array
 
     /**
      * @dataProvider providerParseInvalidStringThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidStringThrowsException(string $text)
+    public function testParseInvalidStringThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         LocalDateTime::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidStringThrowsException() : array
     {
         return [
@@ -144,18 +132,13 @@ public function providerParseInvalidStringThrowsException() : array
 
     /**
      * @dataProvider providerParseInvalidDateTimeThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidDateTimeThrowsException(string $text)
+    public function testParseInvalidDateTimeThrowsException(string $text): void
     {
         $this->expectException(DateTimeException::class);
         LocalDateTime::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidDateTimeThrowsException() : array
     {
         return [
@@ -170,17 +153,17 @@ public function providerParseInvalidDateTimeThrowsException() : array
         ];
     }
 
-    public function testMin()
+    public function testMin(): void
     {
         $this->assertLocalDateTimeIs(Year::MIN_VALUE, 1, 1, 0, 0, 0, 0, LocalDateTime::min());
     }
 
-    public function testMax()
+    public function testMax(): void
     {
         $this->assertLocalDateTimeIs(Year::MAX_VALUE, 12, 31, 23, 59, 59, 999999999, LocalDateTime::max());
     }
 
-    public function testMinMaxOf()
+    public function testMinMaxOf(): void
     {
         $a = LocalDateTime::parse('2003-12-31T12:30:00');
         $b = LocalDateTime::parse('2005-12-31T23:59:59.999999999');
@@ -190,13 +173,13 @@ public function testMinMaxOf()
         $this->assertSame($c, LocalDateTime::maxOf($a, $b, $c));
     }
 
-    public function testMinOfZeroElementsThrowsException()
+    public function testMinOfZeroElementsThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
         LocalDateTime::minOf();
     }
 
-    public function testMaxOfZeroElementsThrowsException()
+    public function testMaxOfZeroElementsThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
         LocalDateTime::maxOf();
@@ -210,15 +193,12 @@ public function testMaxOfZeroElementsThrowsException()
      * @param int $day       The day-of-month to test.
      * @param int $dayOfWeek The expected day-of-week number.
      */
-    public function testGetDayOfWeek(int $year, int $month, int $day, int $dayOfWeek)
+    public function testGetDayOfWeek(int $year, int $month, int $day, int $dayOfWeek): void
     {
         $dateTime = LocalDateTime::of($year, $month, $day, 15, 30, 45);
         $this->assertDayOfWeekIs($dayOfWeek, $dateTime->getDayOfWeek());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetDayOfWeek() : array
     {
         return [
@@ -247,15 +227,12 @@ public function providerGetDayOfWeek() : array
      * @param int $day       The day-of-month to test.
      * @param int $dayOfYear The expected day-of-year number.
      */
-    public function testGetDayOfYear(int $year, int $month, int $day, int $dayOfYear)
+    public function testGetDayOfYear(int $year, int $month, int $day, int $dayOfYear): void
     {
         $dateTime = LocalDate::of($year, $month, $day)->atTime(LocalTime::midnight());
         $this->assertSame($dayOfYear, $dateTime->getDayOfYear());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetDayOfYear() : array
     {
         return [
@@ -270,20 +247,17 @@ public function providerGetDayOfYear() : array
     /**
      * @dataProvider providerWithDate
      *
-     * @param int $y  The new year.
-     * @param int $m  The new month.
-     * @param int $d  The new day.
+     * @param int $y The new year.
+     * @param int $m The new month.
+     * @param int $d The new day.
      */
-    public function testWithDate(int $y, int $m, int $d)
+    public function testWithDate(int $y, int $m, int $d): void
     {
         $dateTime = LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6, 789));
         $newDate = LocalDate::of($y, $m, $d);
         $this->assertLocalDateTimeIs($y, $m, $d, 4, 5, 6, 789, $dateTime->withDate($newDate));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithDate() : array
     {
         return [
@@ -300,16 +274,13 @@ public function providerWithDate() : array
      * @param int $s The new second.
      * @param int $n The new nano.
      */
-    public function testWithTime(int $h, int $m, int $s, int $n)
+    public function testWithTime(int $h, int $m, int $s, int $n): void
     {
         $dateTime = LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6, 789));
         $newTime = LocalTime::of($h, $m, $s, $n);
         $this->assertLocalDateTimeIs(2001, 2, 3, $h, $m, $s, $n, $dateTime->withTime($newTime));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithTime() : array
     {
         return [
@@ -327,7 +298,7 @@ public function providerWithTime() : array
      * @param int $newYear     The new year.
      * @param int $expectedDay The expected day-of-month of the resulting date.
      */
-    public function testWithYear(int $year, int $month, int $day, int $newYear, int $expectedDay)
+    public function testWithYear(int $year, int $month, int $day, int $newYear, int $expectedDay): void
     {
         $date = LocalDate::of($year, $month, $day);
         $time = LocalTime::of(1, 2, 3, 123456789);
@@ -335,9 +306,6 @@ public function testWithYear(int $year, int $month, int $day, int $newYear, int
         $this->assertLocalDateTimeIs($newYear, $month, $expectedDay, 1, 2, 3, 123456789, $localDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithYear() : array
     {
         return [
@@ -355,15 +323,12 @@ public function providerWithYear() : array
      *
      * @param int $invalidYear The year to test.
      */
-    public function testWithInvalidYearThrowsException(int $invalidYear)
+    public function testWithInvalidYearThrowsException(int $invalidYear): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6))->withYear($invalidYear);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidYearThrowsException() : array
     {
         return [
@@ -381,7 +346,7 @@ public function providerWithInvalidYearThrowsException() : array
      * @param int $newMonth    The new month.
      * @param int $expectedDay The expected day-of-month of the resulting date.
      */
-    public function testWithMonth(int $year, int $month, int $day, int $newMonth, int $expectedDay)
+    public function testWithMonth(int $year, int $month, int $day, int $newMonth, int $expectedDay): void
     {
         $date = LocalDate::of($year, $month, $day);
         $time = LocalTime::of(1, 2, 3, 123456789);
@@ -389,9 +354,6 @@ public function testWithMonth(int $year, int $month, int $day, int $newMonth, in
         $this->assertLocalDateTimeIs($year, $newMonth, $expectedDay, 1, 2, 3, 123456789, $localDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithMonth() : array
     {
         return [
@@ -417,15 +379,12 @@ public function providerWithMonth() : array
      *
      * @param int $invalidMonth The month to test.
      */
-    public function testWithInvalidMonthThrowsException(int $invalidMonth)
+    public function testWithInvalidMonthThrowsException(int $invalidMonth): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6))->withMonth($invalidMonth);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidMonthThrowsException() : array
     {
         return [
@@ -442,7 +401,7 @@ public function providerWithInvalidMonthThrowsException() : array
      * @param int $day    The base day-of-month.
      * @param int $newDay The new day-of-month.
      */
-    public function testWithDay(int $year, int $month, int $day, int $newDay)
+    public function testWithDay(int $year, int $month, int $day, int $newDay): void
     {
         $date = LocalDate::of($year, $month, $day);
         $time = LocalTime::of(1, 2, 3, 123456789);
@@ -450,9 +409,6 @@ public function testWithDay(int $year, int $month, int $day, int $newDay)
         $this->assertLocalDateTimeIs($year, $month, $newDay, 1, 2, 3, 123456789, $localDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithDay() : array
     {
         return [
@@ -471,15 +427,12 @@ public function providerWithDay() : array
      * @param int $day    The base day-of-month.
      * @param int $newDay The new day-of-month.
      */
-    public function testWithInvalidDayThrowsException(int $year, int $month, int $day, int $newDay)
+    public function testWithInvalidDayThrowsException(int $year, int $month, int $day, int $newDay): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of($year, $month, $day)->atTime(LocalTime::midnight())->withDay($newDay);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidDayThrowsException() : array
     {
         return [
@@ -496,15 +449,12 @@ public function providerWithInvalidDayThrowsException() : array
      *
      * @param int $hour The new hour.
      */
-    public function testWithHour(int $hour)
+    public function testWithHour(int $hour): void
     {
         $localDateTime = LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(12, 34, 56, 123456789));
         $this->assertLocalDateTimeIs(2001, 2, 3, $hour, 34, 56, 123456789, $localDateTime->withHour($hour));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithHour() : array
     {
         return [
@@ -518,15 +468,12 @@ public function providerWithHour() : array
      *
      * @param int $invalidHour
      */
-    public function testWithInvalidHourThrowsException(int $invalidHour)
+    public function testWithInvalidHourThrowsException(int $invalidHour): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6))->withHour($invalidHour);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidHourThrowsException() : array
     {
         return [
@@ -540,15 +487,12 @@ public function providerWithInvalidHourThrowsException() : array
      *
      * @param int $minute The new minute.
      */
-    public function testWithMinute(int $minute)
+    public function testWithMinute(int $minute): void
     {
         $localDateTime = LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(12, 34, 56, 123456789));
         $this->assertLocalDateTimeIs(2001, 2, 3, 12, $minute, 56, 123456789, $localDateTime->withMinute($minute));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithMinute() : array
     {
         return [
@@ -559,18 +503,13 @@ public function providerWithMinute() : array
 
     /**
      * @dataProvider providerWithInvalidMinuteThrowsException
-     *
-     * @param int $invalidMinute
      */
-    public function testWithInvalidMinuteThrowsException(int $invalidMinute)
+    public function testWithInvalidMinuteThrowsException(int $invalidMinute): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6))->withMinute($invalidMinute);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidMinuteThrowsException() : array
     {
         return [
@@ -584,15 +523,12 @@ public function providerWithInvalidMinuteThrowsException() : array
      *
      * @param int $second The new second.
      */
-    public function testWithSecond(int $second)
+    public function testWithSecond(int $second): void
     {
         $localDateTime = LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(12, 34, 56, 123456789));
         $this->assertLocalDateTimeIs(2001, 2, 3, 12, 34, $second, 123456789, $localDateTime->withSecond($second));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithSecond() : array
     {
         return [
@@ -603,18 +539,13 @@ public function providerWithSecond() : array
 
     /**
      * @dataProvider providerWithInvalidSecondThrowsException
-     *
-     * @param int $invalidSecond
      */
-    public function testWithInvalidSecondThrowsException(int $invalidSecond)
+    public function testWithInvalidSecondThrowsException(int $invalidSecond): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6))->withSecond($invalidSecond);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidSecondThrowsException() : array
     {
         return [
@@ -628,15 +559,12 @@ public function providerWithInvalidSecondThrowsException() : array
      *
      * @param int $nano The new nano.
      */
-    public function testWithNano(int $nano)
+    public function testWithNano(int $nano): void
     {
         $localDateTime = LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(12, 34, 56, 123456789));
         $this->assertLocalDateTimeIs(2001, 2, 3, 12, 34, 56, $nano, $localDateTime->withNano($nano));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithNano() : array
     {
         return [
@@ -647,18 +575,13 @@ public function providerWithNano() : array
 
     /**
      * @dataProvider providerWithInvalidNanoThrowsException
-     *
-     * @param int $invalidNano
      */
-    public function testWithInvalidNanoThrowsException(int $invalidNano)
+    public function testWithInvalidNanoThrowsException(int $invalidNano): void
     {
         $this->expectException(DateTimeException::class);
         LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6))->withNano($invalidNano);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidNanoThrowsException() : array
     {
         return [
@@ -680,7 +603,7 @@ public function providerWithInvalidNanoThrowsException() : array
      * @param int $em The expected month of the result date.
      * @param int $ed The expected day of the result date.
      */
-    public function testPlusPeriod(int $y, int $m, int $d, int $py, int $pm, int $pd, int $ey, int $em, int $ed)
+    public function testPlusPeriod(int $y, int $m, int $d, int $py, int $pm, int $pd, int $ey, int $em, int $ed): void
     {
         $dateTime = LocalDate::of($y, $m, $d)->atTime(LocalTime::of(12, 34, 56, 123456789));
         $period = Period::of($py, $pm, $pd);
@@ -701,7 +624,7 @@ public function testPlusPeriod(int $y, int $m, int $d, int $py, int $pm, int $pd
      * @param int $em The expected month of the result date.
      * @param int $ed The expected day of the result date.
      */
-    public function testMinusPeriod(int $y, int $m, int $d, int $py, int $pm, int $pd, int $ey, int $em, int $ed)
+    public function testMinusPeriod(int $y, int $m, int $d, int $py, int $pm, int $pd, int $ey, int $em, int $ed): void
     {
         $dateTime = LocalDate::of($y, $m, $d)->atTime(LocalTime::of(12, 34, 56, 123456789));
         $period = Period::of($py, $pm, $pd);
@@ -709,9 +632,6 @@ public function testMinusPeriod(int $y, int $m, int $d, int $py, int $pm, int $p
         $this->assertLocalDateTimeIs($ey, $em, $ed, 12, 34, 56, 123456789, $dateTime->minusPeriod($period->negated()));
     }
 
-    /**
-     * @return array
-     */
     public function providerPeriod() : array
     {
         return [
@@ -742,7 +662,7 @@ public function providerPeriod() : array
      * @param int $s  The expected second.
      * @param int $n  The expected nano.
      */
-    public function testPlusDuration(int $ds, int $dn, int $y, int $m, int $d, int $h, int $i, int $s, int $n)
+    public function testPlusDuration(int $ds, int $dn, int $y, int $m, int $d, int $h, int $i, int $s, int $n): void
     {
         $localDateTime = LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6, 123456789));
         $duration = Duration::ofSeconds($ds, $dn);
@@ -762,16 +682,13 @@ public function testPlusDuration(int $ds, int $dn, int $y, int $m, int $d, int $
      * @param int $s  The expected second.
      * @param int $n  The expected nano.
      */
-    public function testMinusDuration(int $ds, int $dn, int $y, int $m, int $d, int $h, int $i, int $s, int $n)
+    public function testMinusDuration(int $ds, int $dn, int $y, int $m, int $d, int $h, int $i, int $s, int $n): void
     {
         $localDateTime = LocalDate::of(2001, 2, 3)->atTime(LocalTime::of(4, 5, 6, 123456789));
         $duration = Duration::ofSeconds(-$ds, -$dn);
         $this->assertLocalDateTimeIs($y, $m, $d, $h, $i, $s, $n, $localDateTime->minusDuration($duration));
     }
 
-    /**
-     * @return array
-     */
     public function providerDuration() : array
     {
         return [
@@ -790,15 +707,12 @@ public function providerDuration() : array
      * @param int    $years            The number of years to add.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testPlusYears(string $dateTime, int $years, string $expectedDateTime)
+    public function testPlusYears(string $dateTime, int $years, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->plusYears($years);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusYears() : array
     {
         return [
@@ -815,15 +729,12 @@ public function providerPlusYears() : array
      * @param int    $months           The number of months to add.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testPlusMonths(string $dateTime, int $months, string $expectedDateTime)
+    public function testPlusMonths(string $dateTime, int $months, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->plusMonths($months);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusMonths() : array
     {
         return [
@@ -840,15 +751,12 @@ public function providerPlusMonths() : array
      * @param int    $weeks            The number of weeks to add.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testPlusWeeks(string $dateTime, int $weeks, string $expectedDateTime)
+    public function testPlusWeeks(string $dateTime, int $weeks, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->plusWeeks($weeks);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusWeeks() : array
     {
         return [
@@ -865,15 +773,12 @@ public function providerPlusWeeks() : array
      * @param int    $days             The number of days to add.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testPlusDays(string $dateTime, int $days, string $expectedDateTime)
+    public function testPlusDays(string $dateTime, int $days, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->plusDays($days);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusDays() : array
     {
         return [
@@ -890,15 +795,12 @@ public function providerPlusDays() : array
      * @param int    $hours            The number of hours to add.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testPlusHours(string $dateTime, int $hours, string $expectedDateTime)
+    public function testPlusHours(string $dateTime, int $hours, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->plusHours($hours);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusHours() : array
     {
         return [
@@ -940,15 +842,12 @@ public function providerPlusMinutes() : array
      * @param int    $seconds          The number of seconds to add.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testPlusSeconds(string $dateTime, int $seconds, string $expectedDateTime)
+    public function testPlusSeconds(string $dateTime, int $seconds, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->plusSeconds($seconds);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusSeconds() : array
     {
         return [
@@ -965,15 +864,12 @@ public function providerPlusSeconds() : array
      * @param int    $nanosToAdd       The nanoseconds to add.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testPlusNanos(string $dateTime, int $nanosToAdd, string $expectedDateTime)
+    public function testPlusNanos(string $dateTime, int $nanosToAdd, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->plusNanos($nanosToAdd);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusNanos() : array
     {
         return [
@@ -991,15 +887,12 @@ public function providerPlusNanos() : array
      * @param int    $years            The number of years to subtract.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testMinusYears(string $dateTime, int $years, string $expectedDateTime)
+    public function testMinusYears(string $dateTime, int $years, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->minusYears($years);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusYears() : array
     {
         return [
@@ -1016,15 +909,12 @@ public function providerMinusYears() : array
      * @param int    $months           The number of months to subtract.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testMinusMonths(string $dateTime, int $months, string $expectedDateTime)
+    public function testMinusMonths(string $dateTime, int $months, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->minusMonths($months);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusMonths() : array
     {
         return [
@@ -1041,15 +931,12 @@ public function providerMinusMonths() : array
      * @param int    $weeks            The number of weeks to subtract.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testMinusWeeks(string $dateTime, int $weeks, string $expectedDateTime)
+    public function testMinusWeeks(string $dateTime, int $weeks, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->minusWeeks($weeks);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusWeeks() : array
     {
         return [
@@ -1066,15 +953,12 @@ public function providerMinusWeeks() : array
      * @param int    $days             The number of days to subtract.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testMinusDays(string $dateTime, int $days, string $expectedDateTime)
+    public function testMinusDays(string $dateTime, int $days, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->minusDays($days);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusDays() : array
     {
         return [
@@ -1091,15 +975,12 @@ public function providerMinusDays() : array
      * @param int    $hours            The number of hours to subtract.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testMinusHours(string $dateTime, int $hours, string $expectedDateTime)
+    public function testMinusHours(string $dateTime, int $hours, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->minusHours($hours);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusHours() : array
     {
         return [
@@ -1116,15 +997,12 @@ public function providerMinusHours() : array
      * @param int    $minutes          The number of minutes to subtract.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testMinusMinutes(string $dateTime, int $minutes, string $expectedDateTime)
+    public function testMinusMinutes(string $dateTime, int $minutes, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->minusMinutes($minutes);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusMinutes() : array
     {
         return [
@@ -1141,15 +1019,12 @@ public function providerMinusMinutes() : array
      * @param int    $seconds          The number of seconds to subtract.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testMinusSeconds(string $dateTime, int $seconds, string $expectedDateTime)
+    public function testMinusSeconds(string $dateTime, int $seconds, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->minusSeconds($seconds);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusSeconds() : array
     {
         return [
@@ -1166,15 +1041,12 @@ public function providerMinusSeconds() : array
      * @param int    $nanosToSubtract  The nanoseconds to subtract.
      * @param string $expectedDateTime The expected resulting date-time string.
      */
-    public function testMinusNanos(string $dateTime, int $nanosToSubtract, string $expectedDateTime)
+    public function testMinusNanos(string $dateTime, int $nanosToSubtract, string $expectedDateTime): void
     {
         $actualDateTime = LocalDateTime::parse($dateTime)->minusNanos($nanosToSubtract);
         $this->assertSame($expectedDateTime, (string) $actualDateTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusNanos() : array
     {
         return [
@@ -1190,18 +1062,15 @@ public function providerMinusNanos() : array
      *
      * @param string $dateTime     The date-time.
      * @param string $timeZone     The time-zone.
-     * @param int    $epochSeconds  The expected epoch second of the resulting instant.
-     * @param int    $nanos The expected nano-of-second of the resulting instant.
+     * @param int    $epochSeconds The expected epoch second of the resulting instant.
+     * @param int    $nanos        The expected nano-of-second of the resulting instant.
      */
-    public function testAtTimeZone(string $dateTime, string $timeZone, int $epochSeconds, int $nanos)
+    public function testAtTimeZone(string $dateTime, string $timeZone, int $epochSeconds, int $nanos): void
     {
         $zonedDateTime = LocalDateTime::parse($dateTime)->atTimeZone(TimeZone::parse($timeZone));
         $this->assertInstantIs($epochSeconds, $nanos, $zonedDateTime->getInstant());
     }
 
-    /**
-     * @return array
-     */
     public function providerAtTimeZone() : array
     {
         return [
@@ -1219,7 +1088,7 @@ public function providerAtTimeZone() : array
      * @param string $dateTime2 The date-time to compare to.
      * @param int    $result    The expected result.
      */
-    public function testCompareTo(string $dateTime1, string $dateTime2, int $result)
+    public function testCompareTo(string $dateTime1, string $dateTime2, int $result): void
     {
         $dateTime1 = LocalDateTime::parse($dateTime1);
         $dateTime2 = LocalDateTime::parse($dateTime2);
@@ -1233,9 +1102,6 @@ public function testCompareTo(string $dateTime1, string $dateTime2, int $result)
         $this->assertSame($result <= 0, $dateTime1->isBeforeOrEqualTo($dateTime2));
     }
 
-    /**
-     * @return array
-     */
     public function providerCompareTo() : array
     {
         return [
@@ -1260,7 +1126,7 @@ public function providerCompareTo() : array
     /**
      * @dataProvider providerForPastFuture
      */
-    public function testIsFuture(int $clockTimestamp, string $localDateTime, string $offset, bool $isFuture)
+    public function testIsFuture(int $clockTimestamp, string $localDateTime, string $offset, bool $isFuture): void
     {
         $clock = new FixedClock(Instant::of($clockTimestamp));
         $localDateTime = LocalDateTime::parse($localDateTime);
@@ -1271,7 +1137,7 @@ public function testIsFuture(int $clockTimestamp, string $localDateTime, string
     /**
      * @dataProvider providerForPastFuture
      */
-    public function testIsPast(int $clockTimestamp, string $localDateTime, string $offset, bool $isFuture)
+    public function testIsPast(int $clockTimestamp, string $localDateTime, string $offset, bool $isFuture): void
     {
         $clock = new FixedClock(Instant::of($clockTimestamp));
         $localDateTime = LocalDateTime::parse($localDateTime);
@@ -1279,10 +1145,7 @@ public function testIsPast(int $clockTimestamp, string $localDateTime, string $o
         $this->assertSame(! $isFuture, $localDateTime->isPast($timeZone, $clock));
     }
 
-    /**
-     * @return array
-     */
-    public function providerForPastFuture()
+    public function providerForPastFuture(): array
     {
         return [
             [1234567890, '2009-02-14T00:31:29', '+01:00', false],
@@ -1298,7 +1161,7 @@ public function providerForPastFuture()
      * @param string $dateTime The date-time string that will be parse()d by LocalDateTime.
      * @param string $expected The expected output from the native DateTime object.
      */
-    public function testToDateTime(string $dateTime, string $expected)
+    public function testToDateTime(string $dateTime, string $expected): void
     {
         $zonedDateTime = LocalDateTime::parse($dateTime);
         $dateTime = $zonedDateTime->toDateTime();
@@ -1313,7 +1176,7 @@ public function testToDateTime(string $dateTime, string $expected)
      * @param string $dateTime The date-time string that will be parse()d by LocalDateTime.
      * @param string $expected The expected output from the native DateTime object.
      */
-    public function testToDateTimeImmutable(string $dateTime, string $expected)
+    public function testToDateTimeImmutable(string $dateTime, string $expected): void
     {
         $zonedDateTime = LocalDateTime::parse($dateTime);
         $dateTime = $zonedDateTime->toDateTimeImmutable();
@@ -1322,10 +1185,7 @@ public function testToDateTimeImmutable(string $dateTime, string $expected)
         $this->assertSame($expected, $dateTime->format('Y-m-d\TH:i:s.uO'));
     }
 
-    /**
-     * @return array
-     */
-    public function providerToDateTime()
+    public function providerToDateTime(): array
     {
         return [
             ['2018-10-18T12:34',              '2018-10-18T12:34:00.000000+0000'],
@@ -1350,7 +1210,7 @@ public function providerToDateTime()
      * @param int    $nano     The nano-of-second.
      * @param string $expected The expected result string.
      */
-    public function testJsonSerialize(int $year, int $month, int $day, int $hour, int $minute, int $second, int $nano, string $expected)
+    public function testJsonSerialize(int $year, int $month, int $day, int $hour, int $minute, int $second, int $nano, string $expected): void
     {
         $this->assertSame(json_encode($expected), json_encode(LocalDateTime::of($year, $month, $day, $hour, $minute, $second, $nano)));
     }
@@ -1367,14 +1227,11 @@ public function testJsonSerialize(int $year, int $month, int $day, int $hour, in
      * @param int    $nano     The nano-of-second.
      * @param string $expected The expected result string.
      */
-    public function testToString(int $year, int $month, int $day, int $hour, int $minute, int $second, int $nano, string $expected)
+    public function testToString(int $year, int $month, int $day, int $hour, int $minute, int $second, int $nano, string $expected): void
     {
         $this->assertSame($expected, (string) LocalDateTime::of($year, $month, $day, $hour, $minute, $second, $nano));
     }
 
-    /**
-     * @return array
-     */
     public function providerToString() : array
     {
         return [
diff --git a/tests/LocalTimeTest.php b/tests/LocalTimeTest.php
index cb61256..f4dafb2 100644
--- a/tests/LocalTimeTest.php
+++ b/tests/LocalTimeTest.php
@@ -18,27 +18,20 @@
  */
 class LocalTimeTest extends AbstractTestCase
 {
-    public function testOf()
+    public function testOf(): void
     {
         $this->assertLocalTimeIs(12, 34, 56, 123456789, LocalTime::of(12, 34, 56, 123456789));
     }
 
     /**
      * @dataProvider providerOfInvalidTimeThrowsException
-     *
-     * @param int $hour
-     * @param int $minute
-     * @param int $second
      */
-    public function testOfInvalidTimeThrowsException(int $hour, int $minute, int $second)
+    public function testOfInvalidTimeThrowsException(int $hour, int $minute, int $second): void
     {
         $this->expectException(DateTimeException::class);
         LocalTime::of($hour, $minute, $second);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInvalidTimeThrowsException() : array
     {
         return [
@@ -59,15 +52,12 @@ public function providerOfInvalidTimeThrowsException() : array
      * @param int $minute       The expected resulting minute.
      * @param int $second       The expected resulting second.
      */
-    public function testOfSecondOfDay(int $secondOfDay, int $hour, int $minute, int $second)
+    public function testOfSecondOfDay(int $secondOfDay, int $hour, int $minute, int $second): void
     {
         $localTime = LocalTime::ofSecondOfDay($secondOfDay, 123);
         $this->assertLocalTimeIs($hour, $minute, $second, 123, $localTime);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfSecondOfDay() : array
     {
         return [
@@ -91,19 +81,13 @@ public function providerOfSecondOfDay() : array
 
     /**
      * @dataProvider providerOfInvalidSecondOfDayThrowsException
-     *
-     * @param int $secondOfDay
-     * @param int $nanoOfSecond
      */
-    public function testOfInvalidSecondOfDayThrowsException(int $secondOfDay, int $nanoOfSecond)
+    public function testOfInvalidSecondOfDayThrowsException(int $secondOfDay, int $nanoOfSecond): void
     {
         $this->expectException(DateTimeException::class);
         LocalTime::ofSecondOfDay($secondOfDay, $nanoOfSecond);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInvalidSecondOfDayThrowsException() : array
     {
         return [
@@ -116,14 +100,8 @@ public function providerOfInvalidSecondOfDayThrowsException() : array
 
     /**
      * @dataProvider providerParse
-     *
-     * @param string $text
-     * @param int    $hour
-     * @param int    $minute
-     * @param int    $second
-     * @param int    $nano
      */
-    public function testParse(string $text, int $hour, int $minute, int $second, int $nano)
+    public function testParse(string $text, int $hour, int $minute, int $second, int $nano): void
     {
         $time = LocalTime::parse($text);
 
@@ -133,9 +111,6 @@ public function testParse(string $text, int $hour, int $minute, int $second, int
         $this->assertSame($nano, $time->getNano());
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -148,18 +123,13 @@ public function providerParse() : array
 
     /**
      * @dataProvider providerParseInvalidStringThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidStringThrowsException(string $text)
+    public function testParseInvalidStringThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         LocalTime::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidStringThrowsException() : array
     {
         return [
@@ -178,7 +148,7 @@ public function providerParseInvalidStringThrowsException() : array
         ];
     }
 
-    public function testFromDateTime()
+    public function testFromDateTime(): void
     {
         $dateTime = new \DateTime('2018-07-21 14:09:10.23456');
         $this->assertLocalTimeIs(14, 9, 10, 234560000, LocalTime::fromDateTime($dateTime));
@@ -195,16 +165,13 @@ public function testFromDateTime()
      * @param int $s      The expected second.
      * @param int $n      The expected nano.
      */
-    public function testNow(int $second, int $nano, int $offset, int $h, int $m, int $s, int $n)
+    public function testNow(int $second, int $nano, int $offset, int $h, int $m, int $s, int $n): void
     {
         $clock = new FixedClock(Instant::of($second, $nano));
         $timeZone = TimeZoneOffset::ofTotalSeconds($offset);
         $this->assertLocalTimeIs($h, $m, $s, $n, LocalTime::now($timeZone, $clock));
     }
 
-    /**
-     * @return array
-     */
     public function providerNow() : array
     {
         return [
@@ -215,22 +182,22 @@ public function providerNow() : array
         ];
     }
 
-    public function testMidnight()
+    public function testMidnight(): void
     {
         $this->assertLocalTimeIs(0, 0, 0, 0, LocalTime::midnight());
     }
 
-    public function testNoon()
+    public function testNoon(): void
     {
         $this->assertLocalTimeIs(12, 0, 0, 0, LocalTime::noon());
     }
 
-    public function testMin()
+    public function testMin(): void
     {
         $this->assertLocalTimeIs(0, 0, 0, 0, LocalTime::min());
     }
 
-    public function testMax()
+    public function testMax(): void
     {
         $this->assertLocalTimeIs(23, 59, 59, 999999999, LocalTime::max());
     }
@@ -240,14 +207,11 @@ public function testMax()
      *
      * @param int $hour The new hour.
      */
-    public function testWithHour(int $hour)
+    public function testWithHour(int $hour): void
     {
         $this->assertLocalTimeIs($hour, 34, 56, 789, LocalTime::of(12, 34, 56, 789)->withHour($hour));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithHour() : array
     {
         return [
@@ -258,18 +222,13 @@ public function providerWithHour() : array
 
     /**
      * @dataProvider providerWithInvalidHourThrowsException
-     *
-     * @param int $invalidHour
      */
-    public function testWithInvalidHourThrowsException(int $invalidHour)
+    public function testWithInvalidHourThrowsException(int $invalidHour): void
     {
         $this->expectException(DateTimeException::class);
         LocalTime::of(12, 34, 56)->withHour($invalidHour);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidHourThrowsException() : array
     {
         return [
@@ -283,14 +242,11 @@ public function providerWithInvalidHourThrowsException() : array
      *
      * @param int $minute The new minute.
      */
-    public function testWithMinute(int $minute)
+    public function testWithMinute(int $minute): void
     {
         $this->assertLocalTimeIs(12, $minute, 56, 789, LocalTime::of(12, 34, 56, 789)->withMinute($minute));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithMinute() : array
     {
         return [
@@ -301,18 +257,13 @@ public function providerWithMinute() : array
 
     /**
      * @dataProvider providerWithInvalidMinuteThrowsException
-     *
-     * @param int $invalidMinute
      */
-    public function testWithInvalidMinuteThrowsException(int $invalidMinute)
+    public function testWithInvalidMinuteThrowsException(int $invalidMinute): void
     {
         $this->expectException(DateTimeException::class);
         LocalTime::of(12, 34, 56)->withMinute($invalidMinute);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidMinuteThrowsException() : array
     {
         return [
@@ -326,14 +277,11 @@ public function providerWithInvalidMinuteThrowsException() : array
      *
      * @param int $second The new second.
      */
-    public function testWithSecond(int $second)
+    public function testWithSecond(int $second): void
     {
         $this->assertLocalTimeIs(12, 34, $second, 789, LocalTime::of(12, 34, 56, 789)->withSecond($second));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithSecond() : array
     {
         return [
@@ -344,18 +292,13 @@ public function providerWithSecond() : array
 
     /**
      * @dataProvider providerWithInvalidSecondThrowsException
-     *
-     * @param int $invalidSecond
      */
-    public function testWithInvalidSecondThrowsException(int $invalidSecond)
+    public function testWithInvalidSecondThrowsException(int $invalidSecond): void
     {
         $this->expectException(DateTimeException::class);
         LocalTime::of(12, 34, 56)->withSecond($invalidSecond);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidSecondThrowsException() : array
     {
         return [
@@ -369,14 +312,11 @@ public function providerWithInvalidSecondThrowsException() : array
      *
      * @param int $nano The new nano.
      */
-    public function testWithNano(int $nano)
+    public function testWithNano(int $nano): void
     {
         $this->assertLocalTimeIs(12, 34, 56, $nano, LocalTime::of(12, 34, 56, 789)->withNano($nano));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithNano() : array
     {
         return [
@@ -387,18 +327,13 @@ public function providerWithNano() : array
 
     /**
      * @dataProvider providerWithInvalidNanoThrowsException
-     *
-     * @param int $invalidNano
      */
-    public function testWithInvalidNanoThrowsException(int $invalidNano)
+    public function testWithInvalidNanoThrowsException(int $invalidNano): void
     {
         $this->expectException(DateTimeException::class);
         LocalTime::of(12, 34, 56)->withNano($invalidNano);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidNanoThrowsException() : array
     {
         return [
@@ -421,7 +356,7 @@ public function providerWithInvalidNanoThrowsException() : array
      * @param int $es The expected second of the result time.
      * @param int $en The expected nano of the result time.
      */
-    public function testPlusDuration(int $h, int $m, int $s, int $n, int $ds, int $dn, int $eh, int $em, int $es, int $en)
+    public function testPlusDuration(int $h, int $m, int $s, int $n, int $ds, int $dn, int $eh, int $em, int $es, int $en): void
     {
         $localTime = LocalTime::of($h, $m, $s, $n);
         $duration = Duration::ofSeconds($ds, $dn);
@@ -442,16 +377,13 @@ public function testPlusDuration(int $h, int $m, int $s, int $n, int $ds, int $d
      * @param int $es The expected second of the result time.
      * @param int $en The expected nano of the result time.
      */
-    public function testMinusDuration(int $h, int $m, int $s, int $n, int $ds, int $dn, int $eh, int $em, int $es, int $en)
+    public function testMinusDuration(int $h, int $m, int $s, int $n, int $ds, int $dn, int $eh, int $em, int $es, int $en): void
     {
         $localTime = LocalTime::of($h, $m, $s, $n);
         $duration = Duration::ofSeconds(-$ds, -$dn);
         $this->assertLocalTimeIs($eh, $em, $es, $en, $localTime->minusDuration($duration));
     }
 
-    /**
-     * @return array
-     */
     public function providerDuration() : array
     {
         return [
@@ -469,7 +401,7 @@ public function providerDuration() : array
      * @param int $d  The number of hours to add.
      * @param int $eh The expected result hour.
      */
-    public function testPlusHours(int $h, int $d, int $eh)
+    public function testPlusHours(int $h, int $d, int $eh): void
     {
         $result = LocalTime::of($h, 34, 56, 789)->plusHours($d);
         $this->assertLocalTimeIs($eh, 34, 56, 789, $result);
@@ -482,15 +414,12 @@ public function testPlusHours(int $h, int $d, int $eh)
      * @param int $d  The number of hours to add.
      * @param int $eh The expected result hour.
      */
-    public function testMinusHours(int $h, int $d, int $eh)
+    public function testMinusHours(int $h, int $d, int $eh): void
     {
         $result = LocalTime::of($h, 34, 56, 789)->minusHours(-$d);
         $this->assertLocalTimeIs($eh, 34, 56, 789, $result);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusHours() : array
     {
         return [
@@ -533,7 +462,7 @@ public function providerPlusHours() : array
      * @param int $eh The expected result hour.
      * @param int $em The expected result minute.
      */
-    public function testPlusMinutes(int $h, int $m, int $d, int $eh, int $em)
+    public function testPlusMinutes(int $h, int $m, int $d, int $eh, int $em): void
     {
         $result = LocalTime::of($h, $m, 56, 789)->plusMinutes($d);
         $this->assertLocalTimeIs($eh, $em, 56, 789, $result);
@@ -548,15 +477,12 @@ public function testPlusMinutes(int $h, int $m, int $d, int $eh, int $em)
      * @param int $eh The expected result hour.
      * @param int $em The expected result minute.
      */
-    public function testMinusMinutes(int $h, int $m, int $d, int $eh, int $em)
+    public function testMinusMinutes(int $h, int $m, int $d, int $eh, int $em): void
     {
         $result = LocalTime::of($h, $m, 56, 789)->minusMinutes(-$d);
         $this->assertLocalTimeIs($eh, $em, 56, 789, $result);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusMinutes() : array
     {
         return [
@@ -619,7 +545,7 @@ public function providerPlusMinutes() : array
      * @param int $em The expected result minute.
      * @param int $es The expected result second.
      */
-    public function testPlusSeconds(int $h, int $m, int $s, int $d, int $eh, int $em, int $es)
+    public function testPlusSeconds(int $h, int $m, int $s, int $d, int $eh, int $em, int $es): void
     {
         $result = LocalTime::of($h, $m, $s, 123456789)->plusSeconds($d);
         $this->assertLocalTimeIs($eh, $em, $es, 123456789, $result);
@@ -636,15 +562,12 @@ public function testPlusSeconds(int $h, int $m, int $s, int $d, int $eh, int $em
      * @param int $em The expected result minute.
      * @param int $es The expected result second.
      */
-    public function testMinusSeconds(int $h, int $m, int $s, int $d, int $eh, int $em, int $es)
+    public function testMinusSeconds(int $h, int $m, int $s, int $d, int $eh, int $em, int $es): void
     {
         $result = LocalTime::of($h, $m, $s, 123456789)->minusSeconds(-$d);
         $this->assertLocalTimeIs($eh, $em, $es, 123456789, $result);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusSeconds() : array
     {
         return [
@@ -721,7 +644,7 @@ public function providerPlusSeconds() : array
      * @param int $es The expected result second.
      * @param int $en The expected result nanosecond.
      */
-    public function testPlusNanos(int $h, int $m, int $s, int $n, int $d, int $eh, int $em, int $es, int $en)
+    public function testPlusNanos(int $h, int $m, int $s, int $n, int $d, int $eh, int $em, int $es, int $en): void
     {
         $result = LocalTime::of($h, $m, $s, $n)->plusNanos($d);
         $this->assertLocalTimeIs($eh, $em, $es, $en, $result);
@@ -740,15 +663,12 @@ public function testPlusNanos(int $h, int $m, int $s, int $n, int $d, int $eh, i
      * @param int $es The expected result second.
      * @param int $en The expected result nanosecond.
      */
-    public function testMinusNanos(int $h, int $m, int $s, int $n, int $d, int $eh, int $em, int $es, int $en)
+    public function testMinusNanos(int $h, int $m, int $s, int $n, int $d, int $eh, int $em, int $es, int $en): void
     {
         $result = LocalTime::of($h, $m, $s, $n)->minusNanos(-$d);
         $this->assertLocalTimeIs($eh, $em, $es, $en, $result);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusNanos() : array
     {
         return [
@@ -789,7 +709,7 @@ public function providerPlusNanos() : array
      * @param int $n2  The nano of the 2nd time.
      * @param int $cmp The comparison value.
      */
-    public function testCompareTo(int $h1, int $m1, int $s1, int $n1, int $h2, int $m2, int $s2, int $n2, int $cmp)
+    public function testCompareTo(int $h1, int $m1, int $s1, int $n1, int $h2, int $m2, int $s2, int $n2, int $cmp): void
     {
         $t1 = LocalTime::of($h1, $m1, $s1, $n1);
         $t2 = LocalTime::of($h2, $m2, $s2, $n2);
@@ -802,9 +722,6 @@ public function testCompareTo(int $h1, int $m1, int $s1, int $n1, int $h2, int $
         $this->assertSame($cmp >= 0, $t1->isAfterOrEqualTo($t2));
     }
 
-    /**
-     * @return array
-     */
     public function providerCompareTo() : array
     {
         return [
@@ -1067,7 +984,7 @@ public function providerCompareTo() : array
         ];
     }
 
-    public function testAtDate()
+    public function testAtDate(): void
     {
         $time = LocalTime::of(12, 34, 56, 789);
         $date = LocalDate::of(2014, 11, 30);
@@ -1077,13 +994,8 @@ public function testAtDate()
 
     /**
      * @dataProvider providerToSecondOfDay
-     *
-     * @param int $hour
-     * @param int $minute
-     * @param int $second
-     * @param int $result
      */
-    public function testToSecondOfDay(int $hour, int $minute, int $second, int $result)
+    public function testToSecondOfDay(int $hour, int $minute, int $second, int $result): void
     {
         $time = LocalTime::of($hour, $minute, $second);
 
@@ -1091,9 +1003,6 @@ public function testToSecondOfDay(int $hour, int $minute, int $second, int $resu
         $this->assertSame($result, $time->withNano(123)->toSecondOfDay());
     }
 
-    /**
-     * @return array
-     */
     public function providerToSecondOfDay() : array
     {
         return [
@@ -1115,7 +1024,7 @@ public function providerToSecondOfDay() : array
      * @param int    $n The nanosecond.
      * @param string $r The expected result.
      */
-    public function testJsonSerialize(int $h, int $m, int $s, int $n, string $r)
+    public function testJsonSerialize(int $h, int $m, int $s, int $n, string $r): void
     {
         $this->assertSame(json_encode($r), json_encode(LocalTime::of($h, $m, $s, $n)));
     }
@@ -1129,14 +1038,11 @@ public function testJsonSerialize(int $h, int $m, int $s, int $n, string $r)
      * @param int    $n The nanosecond.
      * @param string $r The expected result.
      */
-    public function testToString(int $h, int $m, int $s, int $n, string $r)
+    public function testToString(int $h, int $m, int $s, int $n, string $r): void
     {
         $this->assertSame($r, (string) LocalTime::of($h, $m, $s, $n));
     }
 
-    /**
-     * @return array
-     */
     public function providerToString() : array
     {
         return [
@@ -1149,7 +1055,7 @@ public function providerToString() : array
         ];
     }
 
-    public function testMinMaxOf()
+    public function testMinMaxOf(): void
     {
         $a = LocalTime::of(11, 45);
         $b = LocalTime::of(14, 30);
@@ -1159,13 +1065,13 @@ public function testMinMaxOf()
         $this->assertSame($c, LocalTime::maxOf($a, $b, $c));
     }
 
-    public function testMinOfZeroElementsThrowsException()
+    public function testMinOfZeroElementsThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
         LocalTime::minOf();
     }
 
-    public function testMaxOfZeroElementsThrowsException()
+    public function testMaxOfZeroElementsThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
         LocalTime::maxOf();
@@ -1177,7 +1083,7 @@ public function testMaxOfZeroElementsThrowsException()
      * @param string $dateTime The date-time string that will be parse()d by LocalTime.
      * @param string $expected The expected output from the native DateTime object.
      */
-    public function testToDateTime(string $dateTime, string $expected)
+    public function testToDateTime(string $dateTime, string $expected): void
     {
         $zonedDateTime = LocalTime::parse($dateTime);
         $dateTime = $zonedDateTime->toDateTime();
@@ -1192,7 +1098,7 @@ public function testToDateTime(string $dateTime, string $expected)
      * @param string $dateTime The date-time string that will be parse()d by LocalTime.
      * @param string $expected The expected output from the native DateTime object.
      */
-    public function testToDateTimeImmutable(string $dateTime, string $expected)
+    public function testToDateTimeImmutable(string $dateTime, string $expected): void
     {
         $zonedDateTime = LocalTime::parse($dateTime);
         $dateTime = $zonedDateTime->toDateTimeImmutable();
@@ -1201,9 +1107,6 @@ public function testToDateTimeImmutable(string $dateTime, string $expected)
         $this->assertSame($expected, $dateTime->format('Y-m-d\TH:i:s.uO'));
     }
 
-    /**
-     * @return array
-     */
     public function providerToDateTime()
     {
         return [
diff --git a/tests/MonthDayTest.php b/tests/MonthDayTest.php
index ba5209d..43bc941 100644
--- a/tests/MonthDayTest.php
+++ b/tests/MonthDayTest.php
@@ -18,18 +18,12 @@ class MonthDayTest extends AbstractTestCase
 {
     /**
      * @dataProvider providerOf
-     *
-     * @param int $month
-     * @param int $day
      */
-    public function testOf(int $month, int $day)
+    public function testOf(int $month, int $day): void
     {
         $this->assertMonthDayIs($month, $day, MonthDay::of($month, $day));
     }
 
-    /**
-     * @return array
-     */
     public function providerOf() : array
     {
         return [
@@ -51,19 +45,13 @@ public function providerOf() : array
 
     /**
      * @dataProvider providerOfThrowsExceptionOnInvalidMonthDay
-     *
-     * @param int $month
-     * @param int $day
      */
-    public function testOfThrowsExceptionOnInvalidMonthDay(int $month, int $day)
+    public function testOfThrowsExceptionOnInvalidMonthDay(int $month, int $day): void
     {
         $this->expectException(DateTimeException::class);
         MonthDay::of($month, $day);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfThrowsExceptionOnInvalidMonthDay() : array
     {
         return [
@@ -92,14 +80,11 @@ public function providerOfThrowsExceptionOnInvalidMonthDay() : array
      * @param int    $month The expected month.
      * @param int    $day   The expected day.
      */
-    public function testParse(string $text, int $month, int $day)
+    public function testParse(string $text, int $month, int $day): void
     {
         $this->assertMonthDayIs($month, $day, MonthDay::parse($text));
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -111,18 +96,13 @@ public function providerParse() : array
 
     /**
      * @dataProvider providerParseInvalidStringThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidStringThrowsException(string $text)
+    public function testParseInvalidStringThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         MonthDay::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidStringThrowsException() : array
     {
         return [
@@ -143,18 +123,13 @@ public function providerParseInvalidStringThrowsException() : array
 
     /**
      * @dataProvider providerParseInvalidDateThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidDateThrowsException(string $text)
+    public function testParseInvalidDateThrowsException(string $text): void
     {
         $this->expectException(DateTimeException::class);
         MonthDay::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidDateThrowsException() : array
     {
         return [
@@ -173,15 +148,12 @@ public function providerParseInvalidDateThrowsException() : array
      * @param int    $month       The expected month.
      * @param int    $day         The expected day.
      */
-    public function testNow(int $epochSecond, string $timeZone, int $month, int $day)
+    public function testNow(int $epochSecond, string $timeZone, int $month, int $day): void
     {
         $clock = new FixedClock(Instant::of($epochSecond));
         $this->assertMonthDayIs($month, $day, MonthDay::now(TimeZone::parse($timeZone), $clock));
     }
 
-    /**
-     * @return array
-     */
     public function providerNow() : array
     {
         return [
@@ -201,7 +173,7 @@ public function providerNow() : array
      * @param int $d2     The day of the month-day to compare to.
      * @param int $result The expected result.
      */
-    public function testCompareTo(int $m1, int $d1, int $m2, int $d2, int $result)
+    public function testCompareTo(int $m1, int $d1, int $m2, int $d2, int $result): void
     {
         $this->assertSame($result, MonthDay::of($m1, $d1)->compareTo(MonthDay::of($m2, $d2)));
     }
@@ -215,7 +187,7 @@ public function testCompareTo(int $m1, int $d1, int $m2, int $d2, int $result)
      * @param int $d2     The day of the month-day to compare to.
      * @param int $result The expected result.
      */
-    public function testIsEqualTo(int $m1, int $d1, int $m2, int $d2, int $result)
+    public function testIsEqualTo(int $m1, int $d1, int $m2, int $d2, int $result): void
     {
         $this->assertSame($result == 0, MonthDay::of($m1, $d1)->isEqualTo(MonthDay::of($m2, $d2)));
     }
@@ -229,7 +201,7 @@ public function testIsEqualTo(int $m1, int $d1, int $m2, int $d2, int $result)
      * @param int $d2     The day of the month-day to compare to.
      * @param int $result The expected result.
      */
-    public function testIsBefore(int $m1, int $d1, int $m2, int $d2, int $result)
+    public function testIsBefore(int $m1, int $d1, int $m2, int $d2, int $result): void
     {
         $this->assertSame($result == -1, MonthDay::of($m1, $d1)->isBefore(MonthDay::of($m2, $d2)));
     }
@@ -243,14 +215,11 @@ public function testIsBefore(int $m1, int $d1, int $m2, int $d2, int $result)
      * @param int $d2     The day of the month-day to compare to.
      * @param int $result The expected result.
      */
-    public function testIsAfter(int $m1, int $d1, int $m2, int $d2, int $result)
+    public function testIsAfter(int $m1, int $d1, int $m2, int $d2, int $result): void
     {
         $this->assertSame($result == 1, MonthDay::of($m1, $d1)->isAfter(MonthDay::of($m2, $d2)));
     }
 
-    /**
-     * @return array
-     */
     public function providerCompareTo() : array
     {
         return [
@@ -281,14 +250,11 @@ public function providerCompareTo() : array
      * @param int  $year    The year to test against.
      * @param bool $isValid The expected result.
      */
-    public function testIsValidYear(int $month, int $day, int $year, bool $isValid)
+    public function testIsValidYear(int $month, int $day, int $year, bool $isValid): void
     {
         $this->assertSame($isValid, MonthDay::of($month, $day)->isValidYear($year));
     }
 
-    /**
-     * @return array
-     */
     public function providerIsValidYear() : array
     {
         return [
@@ -320,7 +286,7 @@ public function providerIsValidYear() : array
      * @param int $newMonth    The new month to apply.
      * @param int $expectedDay The expected day of the resulting month-day.
      */
-    public function testWithMonth(int $month, int $day, int $newMonth, int $expectedDay)
+    public function testWithMonth(int $month, int $day, int $newMonth, int $expectedDay): void
     {
         $monthDay = MonthDay::of($month, $day);
         $newMonthDay = $monthDay->withMonth($newMonth);
@@ -329,9 +295,6 @@ public function testWithMonth(int $month, int $day, int $newMonth, int $expected
         $this->assertMonthDayIs($newMonth, $expectedDay, $newMonthDay);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithMonth() : array
     {
         return [
@@ -361,15 +324,12 @@ public function providerWithMonth() : array
      * @param int $day      The day of base the month-day to test.
      * @param int $newMonth The new month to apply.
      */
-    public function testWithInvalidMonthThrowsException(int $month, int $day, int $newMonth)
+    public function testWithInvalidMonthThrowsException(int $month, int $day, int $newMonth): void
     {
         $this->expectException(DateTimeException::class);
         MonthDay::of($month, $day)->withMonth($newMonth);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidMonthThrowsException() : array
     {
         return [
@@ -378,7 +338,7 @@ public function providerWithInvalidMonthThrowsException() : array
         ];
     }
 
-    public function testWithDayWithSameDay()
+    public function testWithDayWithSameDay(): void
     {
         $month = 1;
         $day = 20;
@@ -396,7 +356,7 @@ public function testWithDayWithSameDay()
      * @param int $day    The day of base the month-day to test.
      * @param int $newDay The new day to apply.
      */
-    public function testWithDay(int $month, int $day, int $newDay)
+    public function testWithDay(int $month, int $day, int $newDay): void
     {
         $monthDay = MonthDay::of($month, $day);
         $newMonthDay = $monthDay->withDay($newDay);
@@ -405,9 +365,6 @@ public function testWithDay(int $month, int $day, int $newDay)
         $this->assertMonthDayIs($month, $newDay, $newMonthDay);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithDay() : array
     {
         return [
@@ -427,15 +384,12 @@ public function providerWithDay() : array
      * @param int $day      The day of base the month-day to test.
      * @param int $newDay   The new day to apply.
      */
-    public function testWithInvalidDayThrowsException(int $month, int $day, int $newDay)
+    public function testWithInvalidDayThrowsException(int $month, int $day, int $newDay): void
     {
         $this->expectException(DateTimeException::class);
         MonthDay::of($month, $day)->withDay($newDay);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithInvalidDayThrowsException() : array
     {
         return [
@@ -474,14 +428,11 @@ public function providerWithInvalidDayThrowsException() : array
      * @param int $year        The year to combine with the month-day.
      * @param int $expectedDay The expected day of the resulting date.
      */
-    public function testAtYear(int $month, int $day, int $year, int $expectedDay)
+    public function testAtYear(int $month, int $day, int $year, int $expectedDay): void
     {
         $this->assertLocalDateIs($year, $month, $expectedDay, MonthDay::of($month, $day)->atYear($year));
     }
 
-    /**
-     * @return array
-     */
     public function providerAtYear() : array
     {
         return [
@@ -496,18 +447,13 @@ public function providerAtYear() : array
 
     /**
      * @dataProvider providerAtInvalidYearThrowsException
-     *
-     * @param int $year
      */
-    public function testAtInvalidYearThrowsException(int $year)
+    public function testAtInvalidYearThrowsException(int $year): void
     {
         $this->expectException(DateTimeException::class);
         MonthDay::of(1, 1)->atYear($year);
     }
 
-    /**
-     * @return array
-     */
     public function providerAtInvalidYearThrowsException() : array
     {
         return [
@@ -523,7 +469,7 @@ public function providerAtInvalidYearThrowsException() : array
      * @param int    $day    The day of the month-day to test.
      * @param string $string The expected result string.
      */
-    public function testJsonSerialize(int $month, int $day, string $string)
+    public function testJsonSerialize(int $month, int $day, string $string): void
     {
         $this->assertSame(json_encode($string), json_encode(MonthDay::of($month, $day)));
     }
@@ -535,14 +481,11 @@ public function testJsonSerialize(int $month, int $day, string $string)
      * @param int    $day    The day of the month-day to test.
      * @param string $string The expected result string.
      */
-    public function testToString(int $month, int $day, string $string)
+    public function testToString(int $month, int $day, string $string): void
     {
         $this->assertSame($string, (string) MonthDay::of($month, $day));
     }
 
-    /**
-     * @return array
-     */
     public function providerToString() : array
     {
         return [
diff --git a/tests/MonthTest.php b/tests/MonthTest.php
index e623fca..bd13505 100644
--- a/tests/MonthTest.php
+++ b/tests/MonthTest.php
@@ -18,14 +18,11 @@ class MonthTest extends AbstractTestCase
      * @param int $expectedValue The expected value of the constant.
      * @param int $monthConstant The month constant.
      */
-    public function testConstants(int $expectedValue, int $monthConstant)
+    public function testConstants(int $expectedValue, int $monthConstant): void
     {
         $this->assertSame($expectedValue, $monthConstant);
     }
 
-    /**
-     * @return array
-     */
     public function providerConstants() : array
     {
         return [
@@ -44,25 +41,20 @@ public function providerConstants() : array
         ];
     }
 
-    public function testOf()
+    public function testOf(): void
     {
         $this->assertMonthIs(8, Month::of(8));
     }
 
     /**
      * @dataProvider providerOfInvalidMonthThrowsException
-     *
-     * @param int $invalidMonth
      */
-    public function testOfInvalidMonthThrowsException(int $invalidMonth)
+    public function testOfInvalidMonthThrowsException(int $invalidMonth): void
     {
         $this->expectException(DateTimeException::class);
         Month::of($invalidMonth);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInvalidMonthThrowsException() : array
     {
         return [
@@ -72,7 +64,7 @@ public function providerOfInvalidMonthThrowsException() : array
         ];
     }
 
-    public function testGetAll()
+    public function testGetAll(): void
     {
         $currentMonth = Month::JANUARY;
 
@@ -82,7 +74,7 @@ public function testGetAll()
         }
     }
 
-    public function testIs()
+    public function testIs(): void
     {
         for ($i = Month::JANUARY; $i <= Month::DECEMBER; $i++) {
             for ($j = Month::JANUARY; $j <= Month::DECEMBER; $j++) {
@@ -91,7 +83,7 @@ public function testIs()
         }
     }
 
-    public function testIsEqualTo()
+    public function testIsEqualTo(): void
     {
         for ($i = Month::JANUARY; $i <= Month::DECEMBER; $i++) {
             for ($j = Month::JANUARY; $j <= Month::DECEMBER; $j++) {
@@ -106,14 +98,11 @@ public function testIsEqualTo()
      * @param int $month     The month value.
      * @param int $minLength The expected min length.
      */
-    public function testGetMinLength(int $month, int $minLength)
+    public function testGetMinLength(int $month, int $minLength): void
     {
         $this->assertSame($minLength, Month::of($month)->getMinLength());
     }
 
-    /**
-     * @return array
-     */
     public function minLengthProvider() : array
     {
         return [
@@ -138,14 +127,11 @@ public function minLengthProvider() : array
      * @param int $month     The month value.
      * @param int $minLength The expected min length.
      */
-    public function testGetMaxLength(int $month, int $minLength)
+    public function testGetMaxLength(int $month, int $minLength): void
     {
         $this->assertSame($minLength, Month::of($month)->getMaxLength());
     }
 
-    /**
-     * @return array
-     */
     public function maxLengthProvider() : array
     {
         return [
@@ -171,14 +157,11 @@ public function maxLengthProvider() : array
      * @param bool $leapYear       Whether to test on a leap year.
      * @param int  $firstDayOfYear The expected first day of year.
      */
-    public function testFirstDayOfYear(int $month, bool $leapYear, int $firstDayOfYear)
+    public function testFirstDayOfYear(int $month, bool $leapYear, int $firstDayOfYear): void
     {
         $this->assertSame($firstDayOfYear, Month::of($month)->getFirstDayOfYear($leapYear));
     }
 
-    /**
-     * @return array
-     */
     public function providerFirstDayOfYear() : array
     {
         return [
@@ -217,14 +200,11 @@ public function providerFirstDayOfYear() : array
      * @param bool $leapYear       Whether to test on a leap year.
      * @param int  $expectedLength The expected month length.
      */
-    public function testGetLength(int $month, bool $leapYear, int $expectedLength)
+    public function testGetLength(int $month, bool $leapYear, int $expectedLength): void
     {
         $this->assertSame($expectedLength, Month::of($month)->getLength($leapYear));
     }
 
-    /**
-     * @return array
-     */
     public function providerGetLength()
     {
         return [
@@ -256,7 +236,7 @@ public function providerGetLength()
         ];
     }
 
-    public function testPlusMinusEntireYears()
+    public function testPlusMinusEntireYears(): void
     {
         foreach (Month::getAll() as $month) {
             foreach ([-24, -12, 0, 12, 24] as $monthsToAdd) {
@@ -273,7 +253,7 @@ public function testPlusMinusEntireYears()
      * @param int $plusMonths    The number of months to add.
      * @param int $expectedMonth The expected month number.
      */
-    public function testPlus(int $month, int $plusMonths, int $expectedMonth)
+    public function testPlus(int $month, int $plusMonths, int $expectedMonth): void
     {
         $this->assertMonthIs($expectedMonth, Month::of($month)->plus($plusMonths));
     }
@@ -285,14 +265,11 @@ public function testPlus(int $month, int $plusMonths, int $expectedMonth)
      * @param int $plusMonths    The number of months to add.
      * @param int $expectedMonth The expected month number.
      */
-    public function testMinus(int $month, int $plusMonths, int $expectedMonth)
+    public function testMinus(int $month, int $plusMonths, int $expectedMonth): void
     {
         $this->assertMonthIs($expectedMonth, Month::of($month)->minus(-$plusMonths));
     }
 
-    /**
-     * @return \Generator
-     */
     public function providerPlus() : \Generator
     {
         for ($month = Month::JANUARY; $month <= Month::DECEMBER; $month++) {
@@ -317,7 +294,7 @@ public function providerPlus() : \Generator
      * @param int    $month        The month number.
      * @param string $expectedName The expected month name.
      */
-    public function testJsonSerialize(int $month, string $expectedName)
+    public function testJsonSerialize(int $month, string $expectedName): void
     {
         $this->assertSame(json_encode($expectedName), json_encode(Month::of($month)));
     }
@@ -328,14 +305,11 @@ public function testJsonSerialize(int $month, string $expectedName)
      * @param int    $month        The month number.
      * @param string $expectedName The expected month name.
      */
-    public function testToString(int $month, string $expectedName)
+    public function testToString(int $month, string $expectedName): void
     {
         $this->assertSame($expectedName, (string) Month::of($month));
     }
 
-    /**
-     * @return array
-     */
     public function providerToString() : array
     {
         return [
diff --git a/tests/Parser/DateTimeParseResultTest.php b/tests/Parser/DateTimeParseResultTest.php
index 2759230..d54dcfd 100644
--- a/tests/Parser/DateTimeParseResultTest.php
+++ b/tests/Parser/DateTimeParseResultTest.php
@@ -13,7 +13,7 @@
  */
 class DateTimeParseResultTest extends AbstractTestCase
 {
-    public function testGetFieldWithInvalidFieldStringName()
+    public function testGetFieldWithInvalidFieldStringName(): void
     {
         $dateTimeParseResult = new DateTimeParseResult();
 
diff --git a/tests/Parser/PatternParserBuilderTest.php b/tests/Parser/PatternParserBuilderTest.php
index c85f862..4fcbafe 100644
--- a/tests/Parser/PatternParserBuilderTest.php
+++ b/tests/Parser/PatternParserBuilderTest.php
@@ -13,7 +13,7 @@
  */
 class PatternParserBuilderTest extends AbstractTestCase
 {
-    public function testEndOptionalShouldThrowRuntimeException()
+    public function testEndOptionalShouldThrowRuntimeException(): void
     {
         $patternParserBuilder = new PatternParserBuilder();
 
@@ -23,7 +23,7 @@ public function testEndOptionalShouldThrowRuntimeException()
         $patternParserBuilder->endOptional();
     }
 
-    public function testEndGroupShouldThrowRuntimeException()
+    public function testEndGroupShouldThrowRuntimeException(): void
     {
         $patternParserBuilder = new PatternParserBuilder();
 
@@ -33,7 +33,7 @@ public function testEndGroupShouldThrowRuntimeException()
         $patternParserBuilder->endGroup();
     }
 
-    public function testToParserWithNonEmptyStack()
+    public function testToParserWithNonEmptyStack(): void
     {
         $patternParserBuilder = new PatternParserBuilder();
         $patternParserBuilder->startGroup();
diff --git a/tests/PeriodTest.php b/tests/PeriodTest.php
index baad93d..297df67 100644
--- a/tests/PeriodTest.php
+++ b/tests/PeriodTest.php
@@ -13,32 +13,32 @@
  */
 class PeriodTest extends AbstractTestCase
 {
-    public function testOf()
+    public function testOf(): void
     {
         $this->assertPeriodIs(1, 2, 3, Period::of(1, 2, 3));
     }
 
-    public function testOfYears()
+    public function testOfYears(): void
     {
         $this->assertPeriodIs(11, 0, 0, Period::ofYears(11));
     }
 
-    public function testOfMonths()
+    public function testOfMonths(): void
     {
         $this->assertPeriodIs(0, 11, 0, Period::ofMonths(11));
     }
 
-    public function testOfWeeks()
+    public function testOfWeeks(): void
     {
         $this->assertPeriodIs(0, 0, 77, Period::ofWeeks(11));
     }
 
-    public function testOfDays()
+    public function testOfDays(): void
     {
         $this->assertPeriodIs(0, 0, 11, Period::ofDays(11));
     }
 
-    public function testZero()
+    public function testZero(): void
     {
         $this->assertPeriodIs(0, 0, 0, Period::zero());
     }
@@ -51,14 +51,11 @@ public function testZero()
      * @param int    $months The expected months in the period.
      * @param int    $days   The expected days in the period.
      */
-    public function testParse(string $text, int $years, int $months, int $days)
+    public function testParse(string $text, int $years, int $months, int $days): void
     {
         $this->assertPeriodIs($years, $months, $days, Period::parse($text));
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -81,18 +78,13 @@ public function providerParse() : array
 
     /**
      * @dataProvider providerParseInvalidStringThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidStringThrowsException(string $text)
+    public function testParseInvalidStringThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         Period::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidStringThrowsException() : array
     {
         return [
@@ -114,28 +106,28 @@ public function providerParseInvalidStringThrowsException() : array
     /**
      * Extensive testing is done in LocalDate::until().
      */
-    public function testBetween()
+    public function testBetween(): void
     {
         $period = Period::between(LocalDate::of(2010, 1, 15), LocalDate::of(2011, 3, 18));
         $this->assertPeriodIs(1, 2, 3, $period);
     }
 
-    public function testWithYears()
+    public function testWithYears(): void
     {
         $this->assertPeriodIs(9, 2, 3, Period::of(1, 2, 3)->withYears(9));
     }
 
-    public function testWithMonths()
+    public function testWithMonths(): void
     {
         $this->assertPeriodIs(1, 9, 3, Period::of(1, 2, 3)->withMonths(9));
     }
 
-    public function testWithDays()
+    public function testWithDays(): void
     {
         $this->assertPeriodIs(1, 2, 9, Period::of(1, 2, 3)->withDays(9));
     }
 
-    public function testWithSameValuesReturnsThis()
+    public function testWithSameValuesReturnsThis(): void
     {
         $period = Period::of(1, 2, 3);
 
@@ -144,22 +136,22 @@ public function testWithSameValuesReturnsThis()
         $this->assertSame($period, $period->withDays(3));
     }
 
-    public function testPlusYears()
+    public function testPlusYears(): void
     {
         $this->assertPeriodIs(11, 2, 3, Period::of(1, 2, 3)->plusYears(10));
     }
 
-    public function testPlusMonths()
+    public function testPlusMonths(): void
     {
         $this->assertPeriodIs(1, 12, 3, Period::of(1, 2, 3)->plusMonths(10));
     }
 
-    public function testPlusDays()
+    public function testPlusDays(): void
     {
         $this->assertPeriodIs(1, 2, 13, Period::of(1, 2, 3)->plusDays(10));
     }
 
-    public function testPlusZeroReturnsThis()
+    public function testPlusZeroReturnsThis(): void
     {
         $period = Period::of(1, 2, 3);
 
@@ -168,22 +160,22 @@ public function testPlusZeroReturnsThis()
         $this->assertSame($period, $period->plusDays(0));
     }
 
-    public function testMinusYears()
+    public function testMinusYears(): void
     {
         $this->assertPeriodIs(-1, 2, 3, Period::of(1, 2, 3)->minusYears(2));
     }
 
-    public function testMinusMonths()
+    public function testMinusMonths(): void
     {
         $this->assertPeriodIs(1, -2, 3, Period::of(1, 2, 3)->minusMonths(4));
     }
 
-    public function testMinusDays()
+    public function testMinusDays(): void
     {
         $this->assertPeriodIs(1, 2, -3, Period::of(1, 2, 3)->minusDays(6));
     }
 
-    public function testMinusZeroReturnsThis()
+    public function testMinusZeroReturnsThis(): void
     {
         $period = Period::of(1, 2, 3);
 
@@ -192,23 +184,23 @@ public function testMinusZeroReturnsThis()
         $this->assertSame($period, $period->minusDays(0));
     }
 
-    public function testMultipliedBy()
+    public function testMultipliedBy(): void
     {
         $this->assertPeriodIs(-2, -4, -6, Period::of(1, 2, 3)->multipliedBy(-2));
     }
 
-    public function testMultipliedByOneReturnsThis()
+    public function testMultipliedByOneReturnsThis(): void
     {
         $period = Period::of(1, 2, 3);
         $this->assertSame($period, $period->multipliedBy(1));
     }
 
-    public function testNegated()
+    public function testNegated(): void
     {
         $this->assertPeriodIs(-7, -8, -9, Period::of(7, 8, 9)->negated());
     }
 
-    public function testZeroNegatedReturnsThis()
+    public function testZeroNegatedReturnsThis(): void
     {
         $period = Period::zero();
         $this->assertSame($period, $period->negated());
@@ -223,14 +215,11 @@ public function testZeroNegatedReturnsThis()
      * @param int $ny The years of the normalized period.
      * @param int $nm The months of the normalized period.
      */
-    public function testNormalized(int $y, int $m, int $d, int $ny, int $nm)
+    public function testNormalized(int $y, int $m, int $d, int $ny, int $nm): void
     {
         $this->assertPeriodIs($ny, $nm, $d, Period::of($y, $m, $d)->normalized());
     }
 
-    /**
-     * @return array
-     */
     public function providerNormalized() : array
     {
         return [
@@ -253,14 +242,11 @@ public function providerNormalized() : array
      * @param int  $days   The number of days in the period.
      * @param bool $isZero The expected return value.
      */
-    public function testIsZero(int $years, int $months, int $days, bool $isZero)
+    public function testIsZero(int $years, int $months, int $days, bool $isZero): void
     {
         $this->assertSame($isZero, Period::of($years, $months, $days)->isZero());
     }
 
-    /**
-     * @return array
-     */
     public function providerIsZero() : array
     {
         return [
@@ -282,7 +268,7 @@ public function providerIsZero() : array
      * @param int  $d2      The number of days in the 2nd period.
      * @param bool $isEqual The expected return value.
      */
-    public function testIsEqualTo(int $y1, int $m1, int $d1, int $y2, int $m2, int $d2, bool $isEqual)
+    public function testIsEqualTo(int $y1, int $m1, int $d1, int $y2, int $m2, int $d2, bool $isEqual): void
     {
         $p1 = Period::of($y1, $m1, $d1);
         $p2 = Period::of($y2, $m2, $d2);
@@ -291,9 +277,6 @@ public function testIsEqualTo(int $y1, int $m1, int $d1, int $y2, int $m2, int $
         $this->assertSame($isEqual, $p2->isEqualTo($p1));
     }
 
-    /**
-     * @return array
-     */
     public function providerIsEqualTo() : array
     {
         return [
@@ -313,12 +296,8 @@ public function providerIsEqualTo() : array
 
     /**
      * @dataProvider providerToDateInterval
-     *
-     * @param int $years
-     * @param int $months
-     * @param int $days
      */
-    public function testToDateInterval(int $years, int $months, int $days)
+    public function testToDateInterval(int $years, int $months, int $days): void
     {
         $period = Period::of($years, $months, $days);
         $dateInterval = $period->toDateInterval();
@@ -328,9 +307,6 @@ public function testToDateInterval(int $years, int $months, int $days)
         $this->assertSame($days, $dateInterval->d);
     }
 
-    /**
-     * @return array
-     */
     public function providerToDateInterval() : array
     {
         return [
@@ -347,7 +323,7 @@ public function providerToDateInterval() : array
      * @param int    $days     The number of days in the period.
      * @param string $expected The expected string output.
      */
-    public function testJsonSerialize(int $years, int $months, int $days, string $expected)
+    public function testJsonSerialize(int $years, int $months, int $days, string $expected): void
     {
         $this->assertSame(json_encode($expected), json_encode(Period::of($years, $months, $days)));
     }
@@ -360,14 +336,11 @@ public function testJsonSerialize(int $years, int $months, int $days, string $ex
      * @param int    $days     The number of days in the period.
      * @param string $expected The expected string output.
      */
-    public function testToString(int $years, int $months, int $days, string $expected)
+    public function testToString(int $years, int $months, int $days, string $expected): void
     {
         $this->assertSame($expected, (string) Period::of($years, $months, $days));
     }
 
-    /**
-     * @return array
-     */
     public function providerToString() : array
     {
         return [
diff --git a/tests/StopwatchTest.php b/tests/StopwatchTest.php
index 41275f0..c15bfd5 100644
--- a/tests/StopwatchTest.php
+++ b/tests/StopwatchTest.php
@@ -28,7 +28,7 @@ private static function setClockTime(int $second, int $nano) : void
         self::$clock->setTime(Instant::of($second, $nano));
     }
 
-    public function testConstructorWithNullClock()
+    public function testConstructorWithNullClock(): void
     {
         $stopwatch = new Stopwatch();
 
@@ -37,9 +37,6 @@ public function testConstructorWithNullClock()
         $this->assertDurationIs(0, 0, $stopwatch->getElapsedTime());
     }
 
-    /**
-     * @return Stopwatch
-     */
     public function testNew() : Stopwatch
     {
         $stopwatch = new Stopwatch(self::$clock);
@@ -53,10 +50,6 @@ public function testNew() : Stopwatch
 
     /**
      * @depends testNew
-     *
-     * @param Stopwatch $stopwatch
-     *
-     * @return Stopwatch
      */
     public function testStart(Stopwatch $stopwatch) : Stopwatch
     {
@@ -73,10 +66,6 @@ public function testStart(Stopwatch $stopwatch) : Stopwatch
 
     /**
      * @depends testStart
-     *
-     * @param Stopwatch $stopwatch
-     *
-     * @return Stopwatch
      */
     public function testElapsedTimeWhileRunning(Stopwatch $stopwatch) : Stopwatch
     {
@@ -89,7 +78,7 @@ public function testElapsedTimeWhileRunning(Stopwatch $stopwatch) : Stopwatch
         return $stopwatch;
     }
 
-    public function testStopWithNullStartTime()
+    public function testStopWithNullStartTime(): void
     {
         $stopwatch = new Stopwatch();
         $stopwatch->stop();
@@ -101,10 +90,6 @@ public function testStopWithNullStartTime()
 
     /**
      * @depends testElapsedTimeWhileRunning
-     *
-     * @param Stopwatch $stopwatch
-     *
-     * @return Stopwatch
      */
     public function testStop(Stopwatch $stopwatch) : Stopwatch
     {
@@ -121,10 +106,6 @@ public function testStop(Stopwatch $stopwatch) : Stopwatch
 
     /**
      * @depends testStop
-     *
-     * @param Stopwatch $stopwatch
-     *
-     * @return Stopwatch
      */
     public function testFrozenAfterStop(Stopwatch $stopwatch) : Stopwatch
     {
@@ -139,10 +120,6 @@ public function testFrozenAfterStop(Stopwatch $stopwatch) : Stopwatch
 
     /**
      * @depends testFrozenAfterStop
-     *
-     * @param Stopwatch $stopwatch
-     *
-     * @return Stopwatch
      */
     public function testRestart(Stopwatch $stopwatch) : Stopwatch
     {
@@ -159,10 +136,6 @@ public function testRestart(Stopwatch $stopwatch) : Stopwatch
 
     /**
      * @depends testRestart
-     *
-     * @param Stopwatch $stopwatch
-     *
-     * @return Stopwatch
      */
     public function testElapsedTimeWhileRunningAfterRestart(Stopwatch $stopwatch) : Stopwatch
     {
@@ -177,10 +150,6 @@ public function testElapsedTimeWhileRunningAfterRestart(Stopwatch $stopwatch) :
 
     /**
      * @depends testElapsedTimeWhileRunningAfterRestart
-     *
-     * @param Stopwatch $stopwatch
-     *
-     * @return Stopwatch
      */
     public function testStopAgain(Stopwatch $stopwatch) : Stopwatch
     {
@@ -197,10 +166,8 @@ public function testStopAgain(Stopwatch $stopwatch) : Stopwatch
 
     /**
      * @depends testStopAgain
-     *
-     * @param Stopwatch $stopwatch
      */
-    public function testFrozenAfterSecondStop(Stopwatch $stopwatch)
+    public function testFrozenAfterSecondStop(Stopwatch $stopwatch): void
     {
         self::setClockTime(6000, 999);
 
diff --git a/tests/TimeZoneOffsetTest.php b/tests/TimeZoneOffsetTest.php
index f804bce..98de4f8 100644
--- a/tests/TimeZoneOffsetTest.php
+++ b/tests/TimeZoneOffsetTest.php
@@ -22,14 +22,11 @@ class TimeZoneOffsetTest extends AbstractTestCase
      * @param int $seconds      The seconds part of the offset.
      * @param int $totalSeconds The expected total number of seconds.
      */
-    public function testOf(int $hours, int $minutes, int $seconds, int $totalSeconds)
+    public function testOf(int $hours, int $minutes, int $seconds, int $totalSeconds): void
     {
         $this->assertTimeZoneOffsetIs($totalSeconds, TimeZoneOffset::of($hours, $minutes, $seconds));
     }
 
-    /**
-     * @return array
-     */
     public function providerOf() : array
     {
         return [
@@ -62,20 +59,13 @@ public function providerOf() : array
 
     /**
      * @dataProvider providerOfInvalidValuesThrowsException
-     *
-     * @param int $hours
-     * @param int $minutes
-     * @param int $seconds
      */
-    public function testOfInvalidValuesThrowsException(int $hours, int $minutes, int $seconds)
+    public function testOfInvalidValuesThrowsException(int $hours, int $minutes, int $seconds): void
     {
         $this->expectException(DateTimeException::class);
         TimeZoneOffset::of($hours, $minutes, $seconds);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInvalidValuesThrowsException() : array
     {
         return [
@@ -103,17 +93,12 @@ public function providerOfInvalidValuesThrowsException() : array
 
     /**
      * @dataProvider providerTotalSeconds
-     *
-     * @param int $totalSeconds
      */
-    public function testOfTotalSeconds(int $totalSeconds)
+    public function testOfTotalSeconds(int $totalSeconds): void
     {
         $this->assertTimeZoneOffsetIs($totalSeconds, TimeZoneOffset::ofTotalSeconds($totalSeconds));
     }
 
-    /**
-     * @return array
-     */
     public function providerTotalSeconds() : array
     {
         return [
@@ -127,18 +112,13 @@ public function providerTotalSeconds() : array
 
     /**
      * @dataProvider providerOfInvalidTotalSecondsThrowsException
-     *
-     * @param int $totalSeconds
      */
-    public function testOfInvalidTotalSecondsThrowsException(int $totalSeconds)
+    public function testOfInvalidTotalSecondsThrowsException(int $totalSeconds): void
     {
         $this->expectException(DateTimeException::class);
         TimeZoneOffset::ofTotalSeconds($totalSeconds);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInvalidTotalSecondsThrowsException() : array
     {
         return [
@@ -147,7 +127,7 @@ public function providerOfInvalidTotalSecondsThrowsException() : array
         ];
     }
 
-    public function testUtc()
+    public function testUtc(): void
     {
         $this->assertTimeZoneOffsetIs(0, TimeZoneOffset::utc());
     }
@@ -158,14 +138,11 @@ public function testUtc()
      * @param string $text         The text to parse.
      * @param int    $totalSeconds The expected total offset seconds.
      */
-    public function testParse(string $text, int $totalSeconds)
+    public function testParse(string $text, int $totalSeconds): void
     {
         $this->assertTimeZoneOffsetIs($totalSeconds, TimeZoneOffset::parse($text));
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -186,18 +163,13 @@ public function providerParse() : array
 
     /**
      * @dataProvider providerParseInvalidStringThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidStringThrowsException(string $text)
+    public function testParseInvalidStringThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         TimeZoneOffset::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidStringThrowsException() : array
     {
         return [
@@ -214,18 +186,13 @@ public function providerParseInvalidStringThrowsException() : array
 
     /**
      * @dataProvider providerParseValueStringThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidValueThrowsException(string $text)
+    public function testParseInvalidValueThrowsException(string $text): void
     {
         $this->expectException(DateTimeException::class);
         TimeZoneOffset::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseValueStringThrowsException() : array
     {
         return [
@@ -244,7 +211,7 @@ public function providerParseValueStringThrowsException() : array
      * @param int    $totalSeconds The total offset seconds.
      * @param string $expectedId   The expected id.
      */
-    public function testGetId(int $totalSeconds, string $expectedId)
+    public function testGetId(int $totalSeconds, string $expectedId): void
     {
         $this->assertSame($expectedId, TimeZoneOffset::ofTotalSeconds($totalSeconds)->getId());
     }
@@ -255,14 +222,11 @@ public function testGetId(int $totalSeconds, string $expectedId)
      * @param int    $totalSeconds The total offset seconds.
      * @param string $string       The expected string.
      */
-    public function testToString(int $totalSeconds, string $string)
+    public function testToString(int $totalSeconds, string $string): void
     {
         $this->assertSame($string, (string) TimeZoneOffset::ofTotalSeconds($totalSeconds));
     }
 
-    /**
-     * @return array
-     */
     public function providerGetId() : array
     {
         return [
@@ -286,7 +250,7 @@ public function providerGetId() : array
         ];
     }
 
-    public function testGetOffset()
+    public function testGetOffset(): void
     {
         $whateverInstant = Instant::of(123456789, 987654321);
         $timeZoneOffset = TimeZoneOffset::ofTotalSeconds(-18000);
@@ -294,7 +258,7 @@ public function testGetOffset()
         $this->assertSame(-18000, $timeZoneOffset->getOffset($whateverInstant));
     }
 
-    public function testToDateTimeZone()
+    public function testToDateTimeZone(): void
     {
         $dateTimeZone = TimeZoneOffset::ofTotalSeconds(-18000)->toDateTimeZone();
 
diff --git a/tests/TimeZoneRegionTest.php b/tests/TimeZoneRegionTest.php
index 1342f42..95a0817 100644
--- a/tests/TimeZoneRegionTest.php
+++ b/tests/TimeZoneRegionTest.php
@@ -14,25 +14,20 @@
  */
 class TimeZoneRegionTest extends AbstractTestCase
 {
-    public function testOf()
+    public function testOf(): void
     {
         $this->assertSame('Europe/London', TimeZoneRegion::of('Europe/London')->getId());
     }
 
     /**
      * @dataProvider providerOfInvalidRegionThrowsException
-     *
-     * @param string $region
      */
-    public function testOfInvalidRegionThrowsException(string $region)
+    public function testOfInvalidRegionThrowsException(string $region): void
     {
         $this->expectException(DateTimeException::class);
         TimeZoneRegion::of($region);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInvalidRegionThrowsException() : array
     {
         return [
@@ -45,25 +40,20 @@ public function providerOfInvalidRegionThrowsException() : array
         ];
     }
 
-    public function testParse()
+    public function testParse(): void
     {
         $this->assertSame('Europe/London', TimeZoneRegion::parse('Europe/London')->getId());
     }
 
     /**
      * @dataProvider providerParseInvalidStringThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidStringThrowsException(string $text)
+    public function testParseInvalidStringThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         TimeZoneRegion::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidStringThrowsException() : array
     {
         return [
@@ -74,10 +64,8 @@ public function providerParseInvalidStringThrowsException() : array
 
     /**
      * @dataProvider providerGetAllTimeZones
-     *
-     * @param bool $includeObsolete
      */
-    public function testGetAllTimeZones(bool $includeObsolete)
+    public function testGetAllTimeZones(bool $includeObsolete): void
     {
         $identifiers = TimeZoneRegion::getAllIdentifiers($includeObsolete);
         $this->assertGreaterThan(1, \count($identifiers));
@@ -107,9 +95,6 @@ public function testGetAllTimeZones(bool $includeObsolete)
         }
     }
 
-    /**
-     * @return array
-     */
     public function providerGetAllTimeZones() : array
     {
         return [
@@ -120,20 +105,14 @@ public function providerGetAllTimeZones() : array
 
     /**
      * @dataProvider providerGetTimeZonesForCountry
-     *
-     * @param string   $countryCode
-     * @param string[] $expectedIdentifiers
      */
-    public function testGetTimeZonesForCountry(string $countryCode, string ...$expectedIdentifiers)
+    public function testGetTimeZonesForCountry(string $countryCode, string ...$expectedIdentifiers): void
     {
         $identifiers = TimeZoneRegion::getIdentifiersForCountry($countryCode);
 
         $this->assertSame($expectedIdentifiers, $identifiers);
     }
 
-    /**
-     * @return array
-     */
     public function providerGetTimeZonesForCountry() : array
     {
         return [
@@ -156,15 +135,12 @@ public function providerGetTimeZonesForCountry() : array
      * @param int    $epochSecond    The instant to test.
      * @param int    $expectedOffset The expected offset in seconds.
      */
-    public function testGetOffset(string $region, int $epochSecond, int $expectedOffset)
+    public function testGetOffset(string $region, int $epochSecond, int $expectedOffset): void
     {
         $actualOffset = TimeZoneRegion::of($region)->getOffset(Instant::of($epochSecond));
         $this->assertSame($expectedOffset, $actualOffset);
     }
 
-    /**
-     * @return array
-     */
     public function providerGetOffset() : array
     {
         return [
@@ -175,7 +151,7 @@ public function providerGetOffset() : array
         ];
     }
 
-    public function testToDateTimeZone()
+    public function testToDateTimeZone(): void
     {
         $dateTimeZone = TimeZoneRegion::of('Europe/London')->toDateTimeZone();
 
@@ -183,12 +159,12 @@ public function testToDateTimeZone()
         $this->assertSame('Europe/London', $dateTimeZone->getName());
     }
 
-    public function testGetId()
+    public function testGetId(): void
     {
         $this->assertSame('Europe/Paris', TimeZoneRegion::of('Europe/Paris')->getId());
     }
 
-    public function testToString()
+    public function testToString(): void
     {
         $this->assertSame('America/Los_Angeles', (string) TimeZoneRegion::of('America/Los_Angeles'));
     }
diff --git a/tests/TimeZoneTest.php b/tests/TimeZoneTest.php
index cd26df6..4e4709f 100644
--- a/tests/TimeZoneTest.php
+++ b/tests/TimeZoneTest.php
@@ -21,7 +21,7 @@ class TimeZoneTest extends AbstractTestCase
      * @param string $class The expected class name.
      * @param string $id    The expected id.
      */
-    public function testParse(string $text, string $class, string $id)
+    public function testParse(string $text, string $class, string $id): void
     {
         $timeZone = TimeZone::parse($text);
 
@@ -29,9 +29,6 @@ public function testParse(string $text, string $class, string $id)
         $this->assertSame($id, $timeZone->getId());
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -46,18 +43,13 @@ public function providerParse() : array
 
     /**
      * @dataProvider providerParseInvalidStringThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidStringThrowsException(string $text)
+    public function testParseInvalidStringThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         TimeZone::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidStringThrowsException() : array
     {
         return [
@@ -67,12 +59,12 @@ public function providerParseInvalidStringThrowsException() : array
         ];
     }
 
-    public function testUtc()
+    public function testUtc(): void
     {
         $this->assertTimeZoneOffsetIs(0, TimeZone::utc());
     }
 
-    public function testIsEqualTo()
+    public function testIsEqualTo(): void
     {
         $this->assertTrue(TimeZone::utc()->isEqualTo(TimeZoneOffset::ofTotalSeconds(0)));
         $this->assertFalse(TimeZone::utc()->isEqualTo(TimeZoneOffset::ofTotalSeconds(1)));
@@ -83,15 +75,12 @@ public function testIsEqualTo()
      *
      * @param string $tz The time-zone name.
      */
-    public function testFromDateTimeZone(string $tz)
+    public function testFromDateTimeZone(string $tz): void
     {
         $dateTimeZone = new \DateTimeZone($tz);
         $this->assertSame($tz, TimeZone::fromDateTimeZone($dateTimeZone)->getId());
     }
 
-    /**
-     * @return array
-     */
     public function providerFromDateTimeZone() : array
     {
         return [
diff --git a/tests/Utility/MathTest.php b/tests/Utility/MathTest.php
index 61eb655..39cbace 100644
--- a/tests/Utility/MathTest.php
+++ b/tests/Utility/MathTest.php
@@ -20,14 +20,11 @@ class MathTest extends TestCase
      * @param int $b        The divisor.
      * @param int $expected The expected floor division result.
      */
-    public function testFloorDiv(int $a, int $b, int $expected)
+    public function testFloorDiv(int $a, int $b, int $expected): void
     {
         $this->assertSame($expected, Math::floorDiv($a, $b));
     }
 
-    /**
-     * @return array
-     */
     public function providerFloorDiv() : array
     {
         return [
@@ -45,14 +42,11 @@ public function providerFloorDiv() : array
      * @param int $b        The divisor.
      * @param int $expected The expected floor modulus result.
      */
-    public function testFloorMod(int $a, int $b, int $expected)
+    public function testFloorMod(int $a, int $b, int $expected): void
     {
         $this->assertSame($expected, Math::floorMod($a, $b));
     }
 
-    /**
-     * @return array
-     */
     public function providerFloorMod() : array
     {
         return [
diff --git a/tests/YearMonthRangeTest.php b/tests/YearMonthRangeTest.php
index cd548ba..cf97738 100644
--- a/tests/YearMonthRangeTest.php
+++ b/tests/YearMonthRangeTest.php
@@ -14,7 +14,7 @@
  */
 class YearMonthRangeTest extends AbstractTestCase
 {
-    public function testOf()
+    public function testOf(): void
     {
         $this->assertYearMonthRangeIs(2001, 2, 2004, 5, YearMonthRange::of(
             YearMonth::of(2001, 2),
@@ -22,7 +22,7 @@ public function testOf()
         ));
     }
 
-    public function testOfInvalidRangeThrowsException()
+    public function testOfInvalidRangeThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
 
@@ -41,14 +41,11 @@ public function testOfInvalidRangeThrowsException()
      * @param int    $y2   The expected end year.
      * @param int    $m2   The expected end month.
      */
-    public function testParse(string $text, int $y1, int $m1, int $y2, int $m2)
+    public function testParse(string $text, int $y1, int $m1, int $y2, int $m2): void
     {
         $this->assertYearMonthRangeIs($y1, $m1, $y2, $m2, YearMonthRange::parse($text));
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -62,15 +59,12 @@ public function providerParse() : array
      *
      * @param string $text The invalid text to parse.
      */
-    public function testParseInvalidRangeThrowsException(string $text)
+    public function testParseInvalidRangeThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         YearMonthRange::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidRangeThrowsException() : array
     {
         return [
@@ -90,7 +84,7 @@ public function providerParseInvalidRangeThrowsException() : array
      * @param string $testRange The string representation of the range to test.
      * @param bool   $isEqual   Whether this range is expected to be equal to our range.
      */
-    public function testIsEqualTo(string $testRange, bool $isEqual)
+    public function testIsEqualTo(string $testRange, bool $isEqual): void
     {
         $this->assertSame($isEqual, YearMonthRange::of(
             YearMonth::of(2001, 2),
@@ -98,9 +92,6 @@ public function testIsEqualTo(string $testRange, bool $isEqual)
         )->isEqualTo(YearMonthRange::parse($testRange)));
     }
 
-    /**
-     * @return array
-     */
     public function providerIsEqualTo() : array
     {
         return [
@@ -114,19 +105,12 @@ public function providerIsEqualTo() : array
 
     /**
      * @dataProvider providerContains
-     *
-     * @param string $range
-     * @param string $yearMonth
-     * @param bool   $contains
      */
-    public function testContains(string $range, string $yearMonth, bool $contains)
+    public function testContains(string $range, string $yearMonth, bool $contains): void
     {
         $this->assertSame($contains, YearMonthRange::parse($range)->contains(YearMonth::parse($yearMonth)));
     }
 
-    /**
-     * @return array
-     */
     public function providerContains() : array
     {
         return [
@@ -143,7 +127,7 @@ public function providerContains() : array
         ];
     }
 
-    public function testIterator()
+    public function testIterator(): void
     {
         $start = YearMonth::of(2013, 10);
         $end   = YearMonth::of(2014, 3);
@@ -176,14 +160,11 @@ public function testIterator()
      * @param string $range The year-month range string representation.
      * @param int    $count The expected day count.
      */
-    public function testCount(string $range, int $count)
+    public function testCount(string $range, int $count): void
     {
         $this->assertCount($count, YearMonthRange::parse($range));
     }
 
-    /**
-     * @return array
-     */
     public function providerCount() : array
     {
         return [
@@ -195,7 +176,7 @@ public function providerCount() : array
         ];
     }
 
-    public function testJsonSerialize()
+    public function testJsonSerialize(): void
     {
         $this->assertSame(json_encode('2008-12/2011-01'), json_encode(YearMonthRange::of(
             YearMonth::of(2008, 12),
@@ -203,7 +184,7 @@ public function testJsonSerialize()
         )));
     }
 
-    public function testToString()
+    public function testToString(): void
     {
         $this->assertSame('2008-12/2011-01', (string) YearMonthRange::of(
             YearMonth::of(2008, 12),
diff --git a/tests/YearMonthTest.php b/tests/YearMonthTest.php
index 309a3bc..18b0547 100644
--- a/tests/YearMonthTest.php
+++ b/tests/YearMonthTest.php
@@ -15,7 +15,7 @@
  */
 class YearMonthTest extends AbstractTestCase
 {
-    public function testOf()
+    public function testOf(): void
     {
         $this->assertYearMonthIs(2007, 7, YearMonth::of(2007, 7));
     }
@@ -27,14 +27,11 @@ public function testOf()
      * @param int    $year  The expected year.
      * @param int    $month The expected month.
      */
-    public function testParse(string $text, int $year, int $month)
+    public function testParse(string $text, int $year, int $month): void
     {
         $this->assertYearMonthIs($year, $month, YearMonth::parse($text));
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -51,15 +48,12 @@ public function providerParse() : array
      *
      * @param string $text The text to parse.
      */
-    public function testParseInvalidStringThrowsException(string $text)
+    public function testParseInvalidStringThrowsException(string $text): void
     {
         $this->expectException(DateTimeException::class);
         YearMonth::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidStringThrowsException() : array
     {
         return [
@@ -77,15 +71,12 @@ public function providerParseInvalidStringThrowsException() : array
      *
      * @param string $text The text to parse.
      */
-    public function testParseInvalidYearMonthThrowsException(string $text)
+    public function testParseInvalidYearMonthThrowsException(string $text): void
     {
         $this->expectException(DateTimeException::class);
         YearMonth::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidYearMonthThrowsException() : array
     {
         return [
@@ -102,15 +93,12 @@ public function providerParseInvalidYearMonthThrowsException() : array
      * @param int    $year        The expected year.
      * @param int    $month       The expected month.
      */
-    public function testNow(int $epochSecond, string $timeZone, int $year, int $month)
+    public function testNow(int $epochSecond, string $timeZone, int $year, int $month): void
     {
         $clock = new FixedClock(Instant::of($epochSecond));
         $this->assertYearMonthIs($year, $month, YearMonth::now(TimeZone::parse($timeZone), $clock));
     }
 
-    /**
-     * @return array
-     */
     public function providerNow() : array
     {
         return [
@@ -128,14 +116,11 @@ public function providerNow() : array
      * @param int  $month  The month to test.
      * @param bool $isLeap The expected result.
      */
-    public function testIsLeapYear(int $year, int $month, bool $isLeap)
+    public function testIsLeapYear(int $year, int $month, bool $isLeap): void
     {
         $this->assertSame($isLeap, YearMonth::of($year, $month)->isLeapYear());
     }
 
-    /**
-     * @return array
-     */
     public function providerIsLeapYear() : array
     {
         return [
@@ -155,14 +140,11 @@ public function providerIsLeapYear() : array
      * @param int $month  The month.
      * @param int $length The expected length of month.
      */
-    public function testGetLengthOfMonth(int $year, int $month, int $length)
+    public function testGetLengthOfMonth(int $year, int $month, int $length): void
     {
         $this->assertSame($length, YearMonth::of($year, $month)->getLengthOfMonth());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetLengthOfMonth() : array
     {
         return [
@@ -200,14 +182,11 @@ public function providerGetLengthOfMonth() : array
      * @param int $month  The month.
      * @param int $length The expected length of year.
      */
-    public function testGetLengthOfYear(int $year, int $month, int $length)
+    public function testGetLengthOfYear(int $year, int $month, int $length): void
     {
         $this->assertSame($length, YearMonth::of($year, $month)->getLengthOfYear());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetLengthOfYear() : array
     {
         return [
@@ -226,7 +205,7 @@ public function providerGetLengthOfYear() : array
      * @param int $m2     The month of the year-month to compare to.
      * @param int $result The expected result.
      */
-    public function testCompareTo(int $y1, int $m1, int $y2, int $m2, int $result)
+    public function testCompareTo(int $y1, int $m1, int $y2, int $m2, int $result): void
     {
         $this->assertSame($result, YearMonth::of($y1, $m1)->compareTo(YearMonth::of($y2, $m2)));
     }
@@ -240,7 +219,7 @@ public function testCompareTo(int $y1, int $m1, int $y2, int $m2, int $result)
      * @param int $m2     The month of the year-month to compare to.
      * @param int $result The comparison result.
      */
-    public function testIsEqualTo(int $y1, int $m1, int $y2, int $m2, int $result)
+    public function testIsEqualTo(int $y1, int $m1, int $y2, int $m2, int $result): void
     {
         $this->assertSame($result == 0, YearMonth::of($y1, $m1)->isEqualTo(YearMonth::of($y2, $m2)));
     }
@@ -254,7 +233,7 @@ public function testIsEqualTo(int $y1, int $m1, int $y2, int $m2, int $result)
      * @param int $m2     The month of the year-month to compare to.
      * @param int $result The comparison result.
      */
-    public function testIsBefore(int $y1, int $m1, int $y2, int $m2, int $result)
+    public function testIsBefore(int $y1, int $m1, int $y2, int $m2, int $result): void
     {
         $this->assertSame($result == -1, YearMonth::of($y1, $m1)->isBefore(YearMonth::of($y2, $m2)));
     }
@@ -268,7 +247,7 @@ public function testIsBefore(int $y1, int $m1, int $y2, int $m2, int $result)
      * @param int $m2     The month of the year-month to compare to.
      * @param int $result The comparison result.
      */
-    public function testIsBeforeOrEqualTo(int $y1, int $m1, int $y2, int $m2, int $result)
+    public function testIsBeforeOrEqualTo(int $y1, int $m1, int $y2, int $m2, int $result): void
     {
         $this->assertSame($result <= 0, YearMonth::of($y1, $m1)->isBeforeOrEqualTo(YearMonth::of($y2, $m2)));
     }
@@ -282,7 +261,7 @@ public function testIsBeforeOrEqualTo(int $y1, int $m1, int $y2, int $m2, int $r
      * @param int $m2     The month of the year-month to compare to.
      * @param int $result The comparison result.
      */
-    public function testIsAfter(int $y1, int $m1, int $y2, int $m2, int $result)
+    public function testIsAfter(int $y1, int $m1, int $y2, int $m2, int $result): void
     {
         $this->assertSame($result == 1, YearMonth::of($y1, $m1)->isAfter(YearMonth::of($y2, $m2)));
     }
@@ -296,14 +275,11 @@ public function testIsAfter(int $y1, int $m1, int $y2, int $m2, int $result)
      * @param int $m2     The month of the year-month to compare to.
      * @param int $result The comparison result.
      */
-    public function testIsAfterOrEqualTo(int $y1, int $m1, int $y2, int $m2, int $result)
+    public function testIsAfterOrEqualTo(int $y1, int $m1, int $y2, int $m2, int $result): void
     {
         $this->assertSame($result >= 0, YearMonth::of($y1, $m1)->isAfterOrEqualTo(YearMonth::of($y2, $m2)));
     }
 
-    /**
-     * @return array
-     */
     public function providerCompareTo() : array
     {
         return [
@@ -326,46 +302,39 @@ public function providerCompareTo() : array
         ];
     }
 
-    public function testWithYear()
+    public function testWithYear(): void
     {
         $this->assertYearMonthIs(2001, 5, YearMonth::of(2000, 5)->withYear(2001));
     }
 
-    public function testWithYearWithSameYear()
+    public function testWithYearWithSameYear(): void
     {
         $this->assertYearMonthIs(2018, 2, YearMonth::of(2018, 2)->withYear(2018));
     }
 
-    public function testWithMonth()
+    public function testWithMonth(): void
     {
         $this->assertYearMonthIs(2000, 12, YearMonth::of(2000, 1)->withMonth(12));
     }
 
-    public function testWithMonthWithSameMonth()
+    public function testWithMonthWithSameMonth(): void
     {
         $this->assertYearMonthIs(2000, 2, YearMonth::of(2000, 2)->withMonth(2));
     }
 
-    public function testGetFirstDay()
+    public function testGetFirstDay(): void
     {
         $this->assertLocalDateIs(2023, 10, 1, YearMonth::of(2023, 10)->getFirstDay());
     }
 
     /**
      * @dataProvider providerGetLastDay
-     *
-     * @param int $year
-     * @param int $month
-     * @param int $day
      */
-    public function testGetLastDay(int $year, int $month, int $day)
+    public function testGetLastDay(int $year, int $month, int $day): void
     {
         $this->assertLocalDateIs($year, $month, $day, YearMonth::of($year, $month)->getLastDay());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetLastDay() : array
     {
         return [
@@ -377,35 +346,26 @@ public function providerGetLastDay() : array
         ];
     }
 
-    public function testAtDay()
+    public function testAtDay(): void
     {
         $this->assertLocalDateIs(2001, 2, 3, YearMonth::of(2001, 02)->atDay(3));
     }
 
     /**
      * @dataProvider providerPlusYears
-     *
-     * @param int $year
-     * @param int $month
-     * @param int $plusYears
-     * @param int $expectedYear
-     * @param int $expectedMonth
      */
-    public function testPlusYears(int $year, int $month, int $plusYears, int $expectedYear, int $expectedMonth)
+    public function testPlusYears(int $year, int $month, int $plusYears, int $expectedYear, int $expectedMonth): void
     {
         $yearMonth = YearMonth::of($year, $month);
         $this->assertYearMonthIs($expectedYear, $expectedMonth, $yearMonth->plusYears($plusYears));
     }
 
-    public function testPlusZeroYears()
+    public function testPlusZeroYears(): void
     {
         $yearMonth = YearMonth::of(2005, 1);
         $this->assertYearMonthIs(2005, 1, $yearMonth->plusYears(0));
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusYears() : array
     {
         return [
@@ -416,22 +376,13 @@ public function providerPlusYears() : array
 
     /**
      * @dataProvider providerMinusYears
-     *
-     * @param int $year
-     * @param int $month
-     * @param int $plusYears
-     * @param int $expectedYear
-     * @param int $expectedMonth
      */
-    public function testMinusYears(int $year, int $month, int $plusYears, int $expectedYear, int $expectedMonth)
+    public function testMinusYears(int $year, int $month, int $plusYears, int $expectedYear, int $expectedMonth): void
     {
         $yearMonth = YearMonth::of($year, $month);
         $this->assertYearMonthIs($expectedYear, $expectedMonth, $yearMonth->minusYears($plusYears));
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusYears() : array
     {
         return [
@@ -442,22 +393,13 @@ public function providerMinusYears() : array
 
     /**
      * @dataProvider providerPlusMonths
-     *
-     * @param int $year
-     * @param int $month
-     * @param int $plusMonths
-     * @param int $expectedYear
-     * @param int $expectedMonth
      */
-    public function testPlusMonths(int $year, int $month, int $plusMonths, int $expectedYear, int $expectedMonth)
+    public function testPlusMonths(int $year, int $month, int $plusMonths, int $expectedYear, int $expectedMonth): void
     {
         $yearMonth = YearMonth::of($year, $month);
         $this->assertYearMonthIs($expectedYear, $expectedMonth, $yearMonth->plusMonths($plusMonths));
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusMonths() : array
     {
         return [
@@ -475,22 +417,13 @@ public function providerPlusMonths() : array
 
     /**
      * @dataProvider providerMinusMonths
-     *
-     * @param int $year
-     * @param int $month
-     * @param int $plusMonths
-     * @param int $expectedYear
-     * @param int $expectedMonth
      */
-    public function testMinusMonths(int $year, int $month, int $plusMonths, int $expectedYear, int $expectedMonth)
+    public function testMinusMonths(int $year, int $month, int $plusMonths, int $expectedYear, int $expectedMonth): void
     {
         $yearMonth = YearMonth::of($year, $month);
         $this->assertYearMonthIs($expectedYear, $expectedMonth, $yearMonth->minusMonths($plusMonths));
     }
 
-    /**
-     * @return array
-     */
     public function providerMinusMonths() : array
     {
         return [
@@ -506,12 +439,12 @@ public function providerMinusMonths() : array
         ];
     }
 
-    public function testJsonSerialize()
+    public function testJsonSerialize(): void
     {
         $this->assertSame(json_encode('2013-09'), json_encode(YearMonth::of(2013, 9)));
     }
 
-    public function testToString()
+    public function testToString(): void
     {
         $this->assertSame('2013-09', (string) YearMonth::of(2013, 9));
     }
diff --git a/tests/YearTest.php b/tests/YearTest.php
index 659b6b0..ba7f51f 100644
--- a/tests/YearTest.php
+++ b/tests/YearTest.php
@@ -16,25 +16,20 @@
  */
 class YearTest extends AbstractTestCase
 {
-    public function testOf()
+    public function testOf(): void
     {
         $this->assertYearIs(1987, Year::of(1987));
     }
 
     /**
      * @dataProvider providerOfInvalidYearThrowsException
-     *
-     * @param int $invalidYear
      */
-    public function testOfInvalidYearThrowsException(int $invalidYear)
+    public function testOfInvalidYearThrowsException(int $invalidYear): void
     {
         $this->expectException(DateTimeException::class);
         Year::of($invalidYear);
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInvalidYearThrowsException() : array
     {
         return [
@@ -52,15 +47,12 @@ public function providerOfInvalidYearThrowsException() : array
      * @param string $timeZone     The time-zone to get the current year in.
      * @param int    $expectedYear The expected year.
      */
-    public function testNow(int $epochSecond, string $timeZone, int $expectedYear)
+    public function testNow(int $epochSecond, string $timeZone, int $expectedYear): void
     {
         $clock = new FixedClock(Instant::of($epochSecond));
         $this->assertYearIs($expectedYear, Year::now(TimeZone::parse($timeZone), $clock));
     }
 
-    /**
-     * @return array
-     */
     public function providerNow() : array
     {
         return [
@@ -79,14 +71,11 @@ public function providerNow() : array
      * @param int  $year   The year to test.
      * @param bool $isLeap Whether the year is a leap year.
      */
-    public function testIsLeap(int $year, bool $isLeap)
+    public function testIsLeap(int $year, bool $isLeap): void
     {
         $this->assertSame($isLeap, Year::of($year)->isLeap());
     }
 
-    /**
-     * @return array
-     */
     public function providerIsLeap() : array
     {
         return [
@@ -156,14 +145,11 @@ public function providerIsLeap() : array
      * @param int  $day     The day-of-month of the month-day to test.
      * @param bool $isValid Whether the month-day is expected to be valid.
      */
-    public function testIsValidMonthDay(int $year, int $month, int $day, bool $isValid)
+    public function testIsValidMonthDay(int $year, int $month, int $day, bool $isValid): void
     {
         $this->assertSame($isValid, Year::of($year)->isValidMonthDay(MonthDay::of($month, $day)));
     }
 
-    /**
-     * @return array
-     */
     public function providerIsValidMonthDay() : array
     {
         return [
@@ -185,14 +171,11 @@ public function providerIsValidMonthDay() : array
      * @param int $year   The year to test.
      * @param int $length The expected length of year in days.
      */
-    public function testGetLength(int $year, int $length)
+    public function testGetLength(int $year, int $length): void
     {
         $this->assertSame($length, Year::of($year)->getLength());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetLength() : array
     {
         return [
@@ -256,19 +239,12 @@ public function providerGetLength() : array
 
     /**
      * @dataProvider providerPlus
-     *
-     * @param int $year
-     * @param int $plusYears
-     * @param int $expectedYear
      */
-    public function testPlus(int $year, int $plusYears, int $expectedYear)
+    public function testPlus(int $year, int $plusYears, int $expectedYear): void
     {
         $this->assertYearIs($expectedYear, Year::of($year)->plus($plusYears));
     }
 
-    /**
-     * @return array
-     */
     public function providerPlus() : array
     {
         return [
@@ -280,19 +256,12 @@ public function providerPlus() : array
 
     /**
      * @dataProvider providerMinus
-     *
-     * @param int $year
-     * @param int $minusYears
-     * @param int $expectedYear
      */
-    public function testMinus(int $year, int $minusYears, int $expectedYear)
+    public function testMinus(int $year, int $minusYears, int $expectedYear): void
     {
         $this->assertYearIs($expectedYear, Year::of($year)->minus($minusYears));
     }
 
-    /**
-     * @return array
-     */
     public function providerMinus() : array
     {
         return [
@@ -309,7 +278,7 @@ public function providerMinus() : array
      * @param int $year2 The year to compare to.
      * @param int $cmp   The comparison value.
      */
-    public function testCompareTo(int $year1, int $year2, int $cmp)
+    public function testCompareTo(int $year1, int $year2, int $cmp): void
     {
         $this->assertSame($cmp, Year::of($year1)->compareTo(Year::of($year2)));
     }
@@ -321,7 +290,7 @@ public function testCompareTo(int $year1, int $year2, int $cmp)
      * @param int $year2 The year to compare to.
      * @param int $cmp   The comparison value.
      */
-    public function testIsEqualTo(int $year1, int $year2, int $cmp)
+    public function testIsEqualTo(int $year1, int $year2, int $cmp): void
     {
         $this->assertSame($cmp === 0, Year::of($year1)->isEqualTo(Year::of($year2)));
     }
@@ -333,7 +302,7 @@ public function testIsEqualTo(int $year1, int $year2, int $cmp)
      * @param int $year2 The year to compare to.
      * @param int $cmp   The comparison value.
      */
-    public function testIsAfter(int $year1, int $year2, int $cmp)
+    public function testIsAfter(int $year1, int $year2, int $cmp): void
     {
         $this->assertSame($cmp === 1, Year::of($year1)->isAfter(Year::of($year2)));
     }
@@ -345,14 +314,11 @@ public function testIsAfter(int $year1, int $year2, int $cmp)
      * @param int $year2 The year to compare to.
      * @param int $cmp   The comparison value.
      */
-    public function testIsBefore(int $year1, int $year2, int $cmp)
+    public function testIsBefore(int $year1, int $year2, int $cmp): void
     {
         $this->assertSame($cmp === -1, Year::of($year1)->isBefore(Year::of($year2)));
     }
 
-    /**
-     * @return array
-     */
     public function providerCompareTo() : array
     {
         return [
@@ -392,14 +358,11 @@ public function providerCompareTo() : array
      * @param int $month     The expected month of the resulting date.
      * @param int $day       The expected day-of-month of the resulting date.
      */
-    public function testAtDay(int $year, int $dayOfYear, int $month, int $day)
+    public function testAtDay(int $year, int $dayOfYear, int $month, int $day): void
     {
         $this->assertLocalDateIs($year, $month, $day, Year::of($year)->atDay($dayOfYear));
     }
 
-    /**
-     * @return array
-     */
     public function providerAtDay() : array
     {
         return [
@@ -414,31 +377,26 @@ public function providerAtDay() : array
         ];
     }
 
-    public function testAtInvalidDayThrowsException()
+    public function testAtInvalidDayThrowsException(): void
     {
         $this->expectException(DateTimeException::class);
         Year::of(2007)->atDay(366);
     }
 
-    public function testAtMonth()
+    public function testAtMonth(): void
     {
         $this->assertYearMonthIs(2014, 7, Year::of(2014)->atMonth(7));
     }
 
     /**
      * @dataProvider providerAtInvalidMonthThrowsException
-     *
-     * @param int $invalidMonth
      */
-    public function testAtInvalidMonthThrowsException(int $invalidMonth)
+    public function testAtInvalidMonthThrowsException(int $invalidMonth): void
     {
         $this->expectException(DateTimeException::class);
         Year::of(2000)->atMonth($invalidMonth);
     }
 
-    /**
-     * @return array
-     */
     public function providerAtInvalidMonthThrowsException() : array
     {
         return [
@@ -456,15 +414,12 @@ public function providerAtInvalidMonthThrowsException() : array
      * @param int $day         The day-of-month of the month-day to apply.
      * @param int $expectedDay The expected day of the resulting date.
      */
-    public function testAtMonthDay(int $year, int $month, int $day, int $expectedDay)
+    public function testAtMonthDay(int $year, int $month, int $day, int $expectedDay): void
     {
         $monthDay = MonthDay::of($month, $day);
         $this->assertLocalDateIs($year, $month, $expectedDay, Year::of($year)->atMonthDay($monthDay));
     }
 
-    /**
-     * @return array
-     */
     public function providerAtMonthDay() : array
     {
         return [
@@ -478,12 +433,12 @@ public function providerAtMonthDay() : array
         ];
     }
 
-    public function testJsonSerialize()
+    public function testJsonSerialize(): void
     {
         $this->assertSame(json_encode('1987'), json_encode(Year::of(1987)));
     }
 
-    public function testToString()
+    public function testToString(): void
     {
         $this->assertSame('1987', (string) Year::of(1987));
     }
diff --git a/tests/YearWeekTest.php b/tests/YearWeekTest.php
index 5a0b59b..42d8b71 100644
--- a/tests/YearWeekTest.php
+++ b/tests/YearWeekTest.php
@@ -17,9 +17,6 @@
  */
 class YearWeekTest extends AbstractTestCase
 {
-    /**
-     * @return array
-     */
     public function provider53WeekYear() : array
     {
         return [
@@ -99,10 +96,8 @@ public function provider53WeekYear() : array
 
     /**
      * @dataProvider provider53WeekYear
-     *
-     * @param int $year
      */
-    public function testIs53WeekYear(int $year)
+    public function testIs53WeekYear(int $year): void
     {
         $yearWeek = YearWeek::of($year, 1);
         $this->assertTrue($yearWeek->is53WeekYear());
@@ -110,15 +105,8 @@ public function testIs53WeekYear(int $year)
 
     /**
      * @dataProvider providerAtDay
-     *
-     * @param int $weekBasedYear
-     * @param int $weekOfWeekBasedYear
-     * @param int $dayOfWeek
-     * @param int $year
-     * @param int $month
-     * @param int $dayOfMonth
      */
-    public function testAtDay(int $weekBasedYear, int $weekOfWeekBasedYear, int $dayOfWeek, int $year, int $month, int $dayOfMonth)
+    public function testAtDay(int $weekBasedYear, int $weekOfWeekBasedYear, int $dayOfWeek, int $year, int $month, int $dayOfMonth): void
     {
         $yearWeek = YearWeek::of($weekBasedYear, $weekOfWeekBasedYear);
         $actual = $yearWeek->atDay($dayOfWeek);
@@ -126,9 +114,6 @@ public function testAtDay(int $weekBasedYear, int $weekOfWeekBasedYear, int $day
         $this->assertLocalDateIs($year, $month, $dayOfMonth, $actual);
     }
 
-    /**
-     * @return array
-     */
     public function providerAtDay() : array
     {
         return [
@@ -162,7 +147,7 @@ public function providerAtDay() : array
         ];
     }
 
-    public function testAtDayWithInvalidDay()
+    public function testAtDayWithInvalidDay(): void
     {
         $this->expectException(DateTimeException::class);
         $this->expectExceptionMessage('Invalid day-of-week: 0 is not in the range 1 to 7.');
@@ -170,7 +155,7 @@ public function testAtDayWithInvalidDay()
         YearWeek::of(2000, 1)->atDay(0);
     }
 
-    public function testCompareTo()
+    public function testCompareTo(): void
     {
         $years = [-3, -2, 1, 0, 1, 2, 3];
         $weeks = [1, 2, 3, 50, 51, 52];
@@ -205,12 +190,7 @@ public function testCompareTo()
         }
     }
 
-    /**
-     * @param int      $expected
-     * @param YearWeek $a
-     * @param YearWeek $b
-     */
-    private function assertCompareTo(int $expected, YearWeek $a, YearWeek $b)
+    private function assertCompareTo(int $expected, YearWeek $a, YearWeek $b): void
     {
         $this->assertSame($expected, $a->compareTo($b));
         $this->assertSame($expected === -1 || $expected === 0, $a->isBeforeOrEqualTo($b));
@@ -220,9 +200,6 @@ private function assertCompareTo(int $expected, YearWeek $a, YearWeek $b)
         $this->assertSame($expected === 1 || $expected === 0, $a->isAfterOrEqualTo($b));
     }
 
-    /**
-     * @return array
-     */
     public function providerWithYear() : array
     {
         return [
@@ -235,22 +212,13 @@ public function providerWithYear() : array
 
     /**
      * @dataProvider providerWithYear
-     *
-     * @param int $year
-     * @param int $week
-     * @param int $withYear
-     * @param int $expectedYear
-     * @param int $expectedWeek
      */
-    public function testWithYear(int $year, int $week, int $withYear, int $expectedYear, int $expectedWeek)
+    public function testWithYear(int $year, int $week, int $withYear, int $expectedYear, int $expectedWeek): void
     {
         $yearWeek = YearWeek::of($year, $week)->withYear($withYear);
         $this->assertYearWeekIs($expectedYear, $expectedWeek, $yearWeek);
     }
 
-    /**
-     * @return array
-     */
     public function providerWithWeek() : array
     {
         return [
@@ -262,22 +230,13 @@ public function providerWithWeek() : array
 
     /**
      * @dataProvider providerWithWeek
-     *
-     * @param int $year
-     * @param int $week
-     * @param int $withWeek
-     * @param int $expectedYear
-     * @param int $expectedWeek
      */
-    public function testWithWeek(int $year, int $week, int $withWeek, int $expectedYear, int $expectedWeek)
+    public function testWithWeek(int $year, int $week, int $withWeek, int $expectedYear, int $expectedWeek): void
     {
         $yearWeek = YearWeek::of($year, $week)->withWeek($withWeek);
         $this->assertYearWeekIs($expectedYear, $expectedWeek, $yearWeek);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusYears() : array
     {
         return [
@@ -296,14 +255,8 @@ public function providerPlusYears() : array
 
     /**
      * @dataProvider providerPlusYears
-     *
-     * @param int $year
-     * @param int $week
-     * @param int $delta
-     * @param int $expectedYear
-     * @param int $expectedWeek
      */
-    public function testPlusYears(int $year, int $week, int $delta, int $expectedYear, int $expectedWeek)
+    public function testPlusYears(int $year, int $week, int $delta, int $expectedYear, int $expectedWeek): void
     {
         $yearWeek = YearWeek::of($year, $week)->plusYears($delta);
         $this->assertYearWeekIs($expectedYear, $expectedWeek, $yearWeek);
@@ -311,22 +264,13 @@ public function testPlusYears(int $year, int $week, int $delta, int $expectedYea
 
     /**
      * @dataProvider providerPlusYears
-     *
-     * @param int $year
-     * @param int $week
-     * @param int $delta
-     * @param int $expectedYear
-     * @param int $expectedWeek
      */
-    public function testMinusYears(int $year, int $week, int $delta, int $expectedYear, int $expectedWeek)
+    public function testMinusYears(int $year, int $week, int $delta, int $expectedYear, int $expectedWeek): void
     {
         $yearWeek = YearWeek::of($year, $week)->minusYears(-$delta);
         $this->assertYearWeekIs($expectedYear, $expectedWeek, $yearWeek);
     }
 
-    /**
-     * @return array
-     */
     public function providerPlusWeeks() : array
     {
         return [
@@ -348,14 +292,8 @@ public function providerPlusWeeks() : array
 
     /**
      * @dataProvider providerPlusWeeks
-     *
-     * @param int $year
-     * @param int $week
-     * @param int $delta
-     * @param int $expectedYear
-     * @param int $expectedWeek
      */
-    public function testPlusWeeks(int $year, int $week, int $delta, int $expectedYear, int $expectedWeek)
+    public function testPlusWeeks(int $year, int $week, int $delta, int $expectedYear, int $expectedWeek): void
     {
         $yearWeek = YearWeek::of($year, $week)->plusWeeks($delta);
         $this->assertYearWeekIs($expectedYear, $expectedWeek, $yearWeek);
@@ -363,22 +301,13 @@ public function testPlusWeeks(int $year, int $week, int $delta, int $expectedYea
 
     /**
      * @dataProvider providerPlusWeeks
-     *
-     * @param int $year
-     * @param int $week
-     * @param int $delta
-     * @param int $expectedYear
-     * @param int $expectedWeek
      */
-    public function testMinusWeeks(int $year, int $week, int $delta, int $expectedYear, int $expectedWeek)
+    public function testMinusWeeks(int $year, int $week, int $delta, int $expectedYear, int $expectedWeek): void
     {
         $yearWeek = YearWeek::of($year, $week)->minusWeeks(-$delta);
         $this->assertYearWeekIs($expectedYear, $expectedWeek, $yearWeek);
     }
 
-    /**
-     * @return array
-     */
     public function providerToString() : array
     {
         return [
@@ -398,12 +327,8 @@ public function providerToString() : array
 
     /**
      * @dataProvider providerToString
-     *
-     * @param int    $year
-     * @param int    $week
-     * @param string $expected
      */
-    public function testJsonSerialize(int $year, int $week, string $expected)
+    public function testJsonSerialize(int $year, int $week, string $expected): void
     {
         $yearWeek = YearWeek::of($year, $week);
         $this->assertSame(json_encode($expected), json_encode($yearWeek));
@@ -411,18 +336,14 @@ public function testJsonSerialize(int $year, int $week, string $expected)
 
     /**
      * @dataProvider providerToString
-     *
-     * @param int    $year
-     * @param int    $week
-     * @param string $expected
      */
-    public function testToString(int $year, int $week, string $expected)
+    public function testToString(int $year, int $week, string $expected): void
     {
         $yearWeek = YearWeek::of($year, $week);
         $this->assertSame($expected, (string) $yearWeek);
     }
 
-    public function testNow()
+    public function testNow(): void
     {
         $now = new FixedClock(Instant::of(2000000000));
         $timeZone = TimeZone::parse('Asia/Taipei');
@@ -439,7 +360,7 @@ public function testNow()
      * @param string $firstDay The expected first day of the week.
      * @param string $lastDay  The expected last day of the week.
      */
-    public function testGetFirstLastDay(int $year, int $week, string $firstDay, string $lastDay)
+    public function testGetFirstLastDay(int $year, int $week, string $firstDay, string $lastDay): void
     {
         $yearWeek = YearWeek::of($year, $week);
 
@@ -447,9 +368,6 @@ public function testGetFirstLastDay(int $year, int $week, string $firstDay, stri
         $this->assertIs(LocalDate::class, $lastDay, $yearWeek->getLastDay());
     }
 
-    /**
-     * @return array
-     */
     public function providerGetFirstLastDay() : array
     {
         return [
diff --git a/tests/ZonedDateTimeTest.php b/tests/ZonedDateTimeTest.php
index a1f0d75..151b3b4 100644
--- a/tests/ZonedDateTimeTest.php
+++ b/tests/ZonedDateTimeTest.php
@@ -32,7 +32,7 @@ class ZonedDateTimeTest extends AbstractTestCase
      * @param int    $epochSecond   The expected epoch-second the result zoned date-time resolves to.
      * @param int    $nanoOfSecond  The expected nano-of-second of the result zoned date-time.
      */
-    public function testOf(string $localDateTime, string $timeZone, string $offset, int $shift, int $epochSecond, int $nanoOfSecond)
+    public function testOf(string $localDateTime, string $timeZone, string $offset, int $shift, int $epochSecond, int $nanoOfSecond): void
     {
         $localDateTime = LocalDateTime::parse($localDateTime);
         $timeZone = TimeZone::parse($timeZone);
@@ -52,9 +52,6 @@ public function testOf(string $localDateTime, string $timeZone, string $offset,
         $this->assertSame($nanoOfSecond, $zonedDateTime->getNano());
     }
 
-    /**
-     * @return array
-     */
     public function providerOf() : array
     {
         return [
@@ -324,11 +321,8 @@ public function providerOf() : array
 
     /**
      * @dataProvider providerOfInstant
-     *
-     * @param string $formattedDatetime
-     * @param string $timeZone
      */
-    public function testOfInstant(string $formattedDatetime, string $timeZone)
+    public function testOfInstant(string $formattedDatetime, string $timeZone): void
     {
         $instant = Instant::of(1000000000);
         $zonedDateTime = ZonedDateTime::ofInstant($instant, TimeZone::parse($timeZone));
@@ -337,9 +331,6 @@ public function testOfInstant(string $formattedDatetime, string $timeZone)
         $this->assertSame($formattedDatetime, (string) $zonedDateTime->getDateTime());
     }
 
-    /**
-     * @return array
-     */
     public function providerOfInstant() : array
     {
         return [
@@ -357,7 +348,7 @@ public function providerOfInstant() : array
      * @param string $offset The expected time-zone offset.
      * @param string $zone   The expected time-zone, should be the same as offset when no region is specified.
      */
-    public function testParse(string $text, string $date, string $time, string $offset, string $zone)
+    public function testParse(string $text, string $date, string $time, string $offset, string $zone): void
     {
         $zonedDateTime = ZonedDateTime::parse($text);
 
@@ -367,9 +358,6 @@ public function testParse(string $text, string $date, string $time, string $offs
         $this->assertSame($zone, (string) $zonedDateTime->getTimeZone());
     }
 
-    /**
-     * @return array
-     */
     public function providerParse() : array
     {
         return [
@@ -388,18 +376,13 @@ public function providerParse() : array
 
     /**
      * @dataProvider providerParseInvalidStringThrowsException
-     *
-     * @param string $text
      */
-    public function testParseInvalidStringThrowsException(string $text)
+    public function testParseInvalidStringThrowsException(string $text): void
     {
         $this->expectException(DateTimeParseException::class);
         ZonedDateTime::parse($text);
     }
 
-    /**
-     * @return array
-     */
     public function providerParseInvalidStringThrowsException() : array
     {
         return [
@@ -428,20 +411,13 @@ public function providerParseInvalidStringThrowsException() : array
 
     /**
      * @dataProvider providerFromDateTime
-     *
-     * @param string $dateTimeString
-     * @param string $timeZone
-     * @param string $expected
      */
-    public function testFromDateTime(string $dateTimeString, string $timeZone, string $expected)
+    public function testFromDateTime(string $dateTimeString, string $timeZone, string $expected): void
     {
         $dateTime = new \DateTime($dateTimeString, new \DateTimeZone($timeZone));
         $this->assertIs(ZonedDateTime::class, $expected, ZonedDateTime::fromDateTime($dateTime));
     }
 
-    /**
-     * @return array
-     */
     public function providerFromDateTime() : array
     {
         return [
@@ -451,7 +427,7 @@ public function providerFromDateTime() : array
         ];
     }
 
-    public function testChangeTimeZone()
+    public function testChangeTimeZone(): void
     {
         $timezone1 = TimeZone::parse('UTC');
         $timezone2 = TimeZone::parse('America/Los_Angeles');
@@ -477,7 +453,7 @@ public function testChangeTimeZone()
      * @param string $z2  The second zoned date-time.
      * @param int    $cmp The comparison value.
      */
-    public function testCompareTo(string $z1, string $z2, int $cmp)
+    public function testCompareTo(string $z1, string $z2, int $cmp): void
     {
         $z1 = ZonedDateTime::parse($z1);
         $z2 = ZonedDateTime::parse($z2);
@@ -497,9 +473,6 @@ public function testCompareTo(string $z1, string $z2, int $cmp)
         $this->assertSame($cmp <= 0, $z2->isAfterOrEqualTo($z1));
     }
 
-    /**
-     * @return array
-     */
     public function providerCompareTo() : array
     {
         return [
@@ -515,9 +488,6 @@ public function providerCompareTo() : array
         ];
     }
 
-    /**
-     * @return ZonedDateTime
-     */
     private function getTestZonedDateTime() : ZonedDateTime
     {
         $timeZone = TimeZone::parse('America/Los_Angeles');
@@ -526,191 +496,191 @@ private function getTestZonedDateTime() : ZonedDateTime
         return ZonedDateTime::of($localDateTime, $timeZone);
     }
 
-    public function testGetYear()
+    public function testGetYear(): void
     {
         $this->assertSame(2000, $this->getTestZonedDateTime()->getYear());
     }
 
-    public function testGetMonth()
+    public function testGetMonth(): void
     {
         $this->assertSame(1, $this->getTestZonedDateTime()->getMonth());
     }
 
-    public function testGetDay()
+    public function testGetDay(): void
     {
         $this->assertSame(20, $this->getTestZonedDateTime()->getDay());
     }
 
-    public function testGetDayOfWeek()
+    public function testGetDayOfWeek(): void
     {
         $this->assertDayOfWeekIs(DayOfWeek::THURSDAY, $this->getTestZonedDateTime()->getDayOfWeek());
     }
 
-    public function testGetDayOfYear()
+    public function testGetDayOfYear(): void
     {
         $this->assertSame(20, $this->getTestZonedDateTime()->getDayOfYear());
     }
 
-    public function testGetHour()
+    public function testGetHour(): void
     {
         $this->assertSame(12, $this->getTestZonedDateTime()->getHour());
     }
 
-    public function testGetMinute()
+    public function testGetMinute(): void
     {
         $this->assertSame(34, $this->getTestZonedDateTime()->getMinute());
     }
 
-    public function testGetSecond()
+    public function testGetSecond(): void
     {
         $this->assertSame(56, $this->getTestZonedDateTime()->getSecond());
     }
 
-    public function testWithDate()
+    public function testWithDate(): void
     {
         $newDate = LocalDate::of(2000, 1, 22);
 
         $this->assertIs(ZonedDateTime::class, '2000-01-22T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->withDate($newDate));
     }
 
-    public function testWithTime()
+    public function testWithTime(): void
     {
         $time = LocalTime::of(1, 2, 3, 987654321);
 
         $this->assertIs(ZonedDateTime::class, '2000-01-20T01:02:03.987654321-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->withTime($time));
     }
 
-    public function testWithYear()
+    public function testWithYear(): void
     {
         $this->assertIs(ZonedDateTime::class, '2020-01-20T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->withYear(2020));
     }
 
-    public function testWithMonth()
+    public function testWithMonth(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-07-20T12:34:56.123456789-07:00[America/Los_Angeles]', $this->getTestZonedDateTime()->withMonth(7));
     }
 
-    public function testWithDay()
+    public function testWithDay(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-31T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->withDay(31));
     }
 
-    public function testWithHour()
+    public function testWithHour(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T23:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->withHour(23));
     }
 
-    public function testWithMinute()
+    public function testWithMinute(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T12:00:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->withMinute(0));
     }
 
-    public function testWithSecond()
+    public function testWithSecond(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T12:34:06.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->withSecond(6));
     }
 
-    public function testWithNano()
+    public function testWithNano(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T12:34:56.000000123-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->withNano(123));
     }
 
-    public function testWithFixedOffsetTimeZone()
+    public function testWithFixedOffsetTimeZone(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T12:34:56.123456789-08:00', $this->getTestZonedDateTime()->withFixedOffsetTimeZone());
     }
 
-    public function testPlusPeriod()
+    public function testPlusPeriod(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-04-06T12:34:56.123456789-07:00[America/Los_Angeles]', $this->getTestZonedDateTime()->plusPeriod(Period::ofWeeks(11)));
     }
 
-    public function testPlusDuration()
+    public function testPlusDuration(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T12:35:01.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->plusDuration(Duration::ofSeconds(5)));
     }
 
-    public function testPlusYears()
+    public function testPlusYears(): void
     {
         $this->assertIs(ZonedDateTime::class, '2002-01-20T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->plusYears(2));
     }
 
-    public function testPlusMonths()
+    public function testPlusMonths(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-03-20T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->plusMonths(2));
     }
 
-    public function testPlusWeeks()
+    public function testPlusWeeks(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-02-03T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->plusWeeks(2));
     }
 
-    public function testPlusDays()
+    public function testPlusDays(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-22T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->plusDays(2));
     }
 
-    public function testPlusHours()
+    public function testPlusHours(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T14:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->plusHours(2));
     }
 
-    public function testPlusMinutes()
+    public function testPlusMinutes(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T12:36:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->plusMinutes(2));
     }
 
-    public function testPlusSeconds()
+    public function testPlusSeconds(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T12:34:58.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->plusSeconds(2));
     }
 
-    public function testMinusPeriod()
+    public function testMinusPeriod(): void
     {
         $this->assertIs(ZonedDateTime::class, '1999-11-04T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->minusPeriod(Period::ofWeeks(11)));
     }
 
-    public function testMinusDuration()
+    public function testMinusDuration(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T12:34:51.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->minusDuration(Duration::ofSeconds(5)));
     }
 
-    public function testMinusYears()
+    public function testMinusYears(): void
     {
         $this->assertIs(ZonedDateTime::class, '1999-01-20T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->minusYears(1));
     }
 
-    public function testMinusMonths()
+    public function testMinusMonths(): void
     {
         $this->assertIs(ZonedDateTime::class, '1999-12-20T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->minusMonths(1));
     }
 
-    public function testMinusWeeks()
+    public function testMinusWeeks(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-06T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->minusWeeks(2));
     }
 
-    public function testMinusDays()
+    public function testMinusDays(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-18T12:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->minusDays(2));
     }
 
-    public function testMinusHours()
+    public function testMinusHours(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T10:34:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->minusHours(2));
     }
 
-    public function testMinusMinutes()
+    public function testMinusMinutes(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T12:32:56.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->minusMinutes(2));
     }
 
-    public function testMinusSeconds()
+    public function testMinusSeconds(): void
     {
         $this->assertIs(ZonedDateTime::class, '2000-01-20T12:34:54.123456789-08:00[America/Los_Angeles]', $this->getTestZonedDateTime()->minusSeconds(2));
     }
 
-    public function testIsBetweenInclusive()
+    public function testIsBetweenInclusive(): void
     {
         $timeZone = TimeZone::parse('America/Los_Angeles');
         $localDateTime = '2000-01-20T12:34:56.123456789';
@@ -729,7 +699,7 @@ public function testIsBetweenInclusive()
         $this->assertFalse($fromZonedDateTime->isBetweenInclusive($toZonedDateTime, $notIncluZonedDateTime));
     }
 
-    public function testIsBetweenExclusive()
+    public function testIsBetweenExclusive(): void
     {
         $timeZone = TimeZone::parse('America/Los_Angeles');
         $localDateTime = '2000-01-20T12:34:56.123456789';
@@ -751,7 +721,7 @@ public function testIsBetweenExclusive()
     /**
      * @dataProvider providerForPastFuture
      */
-    public function testIsFuture($clockTimestamp, $zonedDateTime, $isFuture)
+    public function testIsFuture(int $clockTimestamp, string $zonedDateTime, bool $isFuture): void
     {
         $clock = new FixedClock(Instant::of($clockTimestamp));
         $zonedDateTime = ZonedDateTime::parse($zonedDateTime);
@@ -761,17 +731,14 @@ public function testIsFuture($clockTimestamp, $zonedDateTime, $isFuture)
     /**
      * @dataProvider providerForPastFuture
      */
-    public function testIsPast($clockTimestamp, $zonedDateTime, $isFuture)
+    public function testIsPast(int $clockTimestamp, string $zonedDateTime, bool $isFuture): void
     {
         $clock = new FixedClock(Instant::of($clockTimestamp));
         $zonedDateTime = ZonedDateTime::parse($zonedDateTime);
         $this->assertSame(! $isFuture, $zonedDateTime->isPast($clock));
     }
 
-    /**
-     * @return array
-     */
-    public function providerForPastFuture()
+    public function providerForPastFuture(): array
     {
         return [
             [1234567890, '2009-02-14T00:31:29+01:00', false],
@@ -787,7 +754,7 @@ public function providerForPastFuture()
      * @param string $dateTime The date-time string that will be parse()d by ZonedDateTime.
      * @param string $expected The expected output from the native DateTime object.
      */
-    public function testToDateTime(string $dateTime, string $expected)
+    public function testToDateTime(string $dateTime, string $expected): void
     {
         $zonedDateTime = ZonedDateTime::parse($dateTime);
         $dateTime = $zonedDateTime->toDateTime();
@@ -802,7 +769,7 @@ public function testToDateTime(string $dateTime, string $expected)
      * @param string $dateTime The date-time string that will be parse()d by ZonedDateTime.
      * @param string $expected The expected output from the native DateTime object.
      */
-    public function testToDateTimeImmutable(string $dateTime, string $expected)
+    public function testToDateTimeImmutable(string $dateTime, string $expected): void
     {
         $zonedDateTime = ZonedDateTime::parse($dateTime);
         $dateTime = $zonedDateTime->toDateTimeImmutable();
@@ -811,10 +778,7 @@ public function testToDateTimeImmutable(string $dateTime, string $expected)
         $this->assertSame($expected, $dateTime->format('Y-m-d\TH:i:s.uO'));
     }
 
-    /**
-     * @return array
-     */
-    public function providerToDateTime()
+    public function providerToDateTime(): array
     {
         return [
             ['2018-10-18T12:34Z',                        '2018-10-18T12:34:00.000000+0000'],
@@ -828,7 +792,7 @@ public function providerToDateTime()
         ];
     }
 
-    public function testJsonSerialize()
+    public function testJsonSerialize(): void
     {
         $timeZone = TimeZone::parse('America/Los_Angeles');
         $localDateTime = '2000-01-20T12:34:56.123456789';
@@ -838,7 +802,7 @@ public function testJsonSerialize()
         $this->assertSame(json_encode('2000-01-20T12:34:56.123456789-08:00[America/Los_Angeles]'), json_encode($zonedDateTime));
     }
 
-    public function testToString()
+    public function testToString(): void
     {
         $timeZone = TimeZone::parse('America/Los_Angeles');
         $localDateTime = '2000-01-20T12:34:56.123456789';