Add clock implementation

src/Clock/Clock.php

@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DigitalCraftsman\DateTimePrecision\Clock;
+
+use DigitalCraftsman\DateTimePrecision\Moment;
+
+interface Clock
+{
+    public function now(): Moment;
+}

src/Clock/FrozenClock.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DigitalCraftsman\DateTimePrecision\Clock;
+
+use DigitalCraftsman\DateTimePrecision\Moment;
+
+final class FrozenClock implements Clock
+{
+    private Moment $moment;
+
+    public function __construct(?Moment $moment = null)
+    {
+        $this->moment = $moment ?? Moment::fromDateTime(new \DateTimeImmutable('now'));
+    }
+
+    public function freeze(Moment $moment): void
+    {
+        $this->moment = $moment;
+    }
+
+    #[\Override]
+    public function now(): Moment
+    {
+        return $this->moment;
+    }
+}

src/Clock/SystemClock.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DigitalCraftsman\DateTimePrecision\Clock;
+
+use DigitalCraftsman\DateTimePrecision\Moment;
+
+final readonly class SystemClock implements Clock
+{
+    #[\Override]
+    public function now(): Moment
+    {
+        return Moment::fromDateTime(new \DateTimeImmutable(
+            'now',
+            new \DateTimeZone('UTC'),
+        ));
+    }
+}