From 14f648f45b5837966f6af0c105fcfcf01246d3a3 Mon Sep 17 00:00:00 2001
From: mikee47 <mike@silyhouse.net>
Date: Sun, 28 Apr 2024 09:23:41 +0100
Subject: [PATCH 1/8] Don't restrict SystemClock timezone offset range

Can vary much more than plus/minus 12 hours
---
 Sming/Core/SystemClock.cpp |  7 ++-----
 Sming/Core/SystemClock.h   | 17 +++++++++++------
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/Sming/Core/SystemClock.cpp b/Sming/Core/SystemClock.cpp
index e109a3aa43..a59b24cec2 100644
--- a/Sming/Core/SystemClock.cpp
+++ b/Sming/Core/SystemClock.cpp
@@ -46,9 +46,6 @@ String SystemClockClass::getSystemTimeString(TimeZone timeType) const
 
 bool SystemClockClass::setTimeZoneOffset(int seconds)
 {
-	if((unsigned)abs(seconds) <= (12 * SECS_PER_HOUR)) {
-		timeZoneOffsetSecs = seconds;
-		return true;
-	}
-	return false;
+	timeZoneOffsetSecs = seconds;
+	return true;
 }
diff --git a/Sming/Core/SystemClock.h b/Sming/Core/SystemClock.h
index 1a79fa49c7..e2f4a732b5 100644
--- a/Sming/Core/SystemClock.h
+++ b/Sming/Core/SystemClock.h
@@ -58,18 +58,23 @@ class SystemClockClass
 	String getSystemTimeString(TimeZone timeType = eTZ_Local) const;
 
 	/** @brief  Sets the local time zone offset
-     *  @param  seconds Offset from UTC of local time zone in seconds (-720 < offset < +720)
-     *  @retval bool true on success, false if value out of range
+     *  @param  seconds Offset from UTC of local time zone in seconds
+	 *  @see    See `setTimeZone()`.
      */
-	bool setTimeZoneOffset(int seconds);
+	void setTimeZoneOffset(int seconds)
+	{
+		timeZoneOffsetSecs = seconds / SECS_PER_MIN;
+	}
 
 	/** @brief Set the local time zone offset in hours
-     *  @param localTimezoneOffset Offset from UTC of local time zone in hours (-12.0 < offset < +12.0)
+     *  @param localTimezoneOffset Offset from UTC of local time zone in hours
      *  @retval bool true on success, false if value out of range
+	 *  @note Values for local standard time may exceed +/- 12
+	 *  For example, Pacific/Kiritimati has a 14-hour offset (in 2024).
 	 */
-	bool setTimeZone(float localTimezoneOffset)
+	void setTimeZone(float localTimezoneOffset)
 	{
-		return setTimeZoneOffset(localTimezoneOffset * SECS_PER_HOUR);
+		setTimeZoneOffset(localTimezoneOffset * SECS_PER_HOUR);
 	}
 
 	/** @brief Get the current time zone offset

From 9aa850df1cb7a3bd756934d795821a0ffed2f46c Mon Sep 17 00:00:00 2001
From: mikee47 <mike@silyhouse.net>
Date: Tue, 28 May 2024 11:22:21 +0100
Subject: [PATCH 2/8] Make core DateTime defines signed, add MINS_PER_HOUR

Causes all sorts of hidden issues performing arithmetic otherwise
---
 Sming/Core/DateTime.h | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/Sming/Core/DateTime.h b/Sming/Core/DateTime.h
index cceb7b28ca..cfc89bb952 100644
--- a/Sming/Core/DateTime.h
+++ b/Sming/Core/DateTime.h
@@ -22,13 +22,14 @@
 #include <sming_attr.h>
 
 /* Useful Constants */
-#define SECS_PER_MIN (60UL)
-#define SECS_PER_HOUR (3600UL)
-#define SECS_PER_DAY (SECS_PER_HOUR * 24L)
-#define DAYS_PER_WEEK (7L)
+#define SECS_PER_MIN 60
+#define SECS_PER_HOUR 3600
+#define SECS_PER_DAY (SECS_PER_HOUR * 24)
+#define MINS_PER_HOUR 60
+#define DAYS_PER_WEEK 7
 #define SECS_PER_WEEK (SECS_PER_DAY * DAYS_PER_WEEK)
-#define SECS_PER_YEAR (SECS_PER_WEEK * 52L)
-#define SECS_YR_2000 (946681200UL)
+#define SECS_PER_YEAR (SECS_PER_WEEK * 52)
+#define SECS_YR_2000 946681200
 
 /** @brief  Days of week
 */

From 56efb093322fd15fea103b3e9ab8950b7c21a67a Mon Sep 17 00:00:00 2001
From: mikee47 <mike@silyhouse.net>
Date: Mon, 27 May 2024 13:09:17 +0100
Subject: [PATCH 3/8] Add timezone support to DateTime and SystemClockClass

This uses `DateTime::ZoneInfo` and `ZonedTime` to provide a basic mechanism for dealing with zoned time information.
Handling rules, etc. must be handled with separate library.
---
 Sming/Core/DateTime.cpp                 |  94 ++++++++++++++--
 Sming/Core/DateTime.h                   |  86 +++++++++++++--
 Sming/Core/SystemClock.cpp              |  20 ++--
 Sming/Core/SystemClock.h                |  31 +++++-
 Sming/Core/ZonedTime.cpp                |   3 +
 Sming/Core/ZonedTime.h                  | 139 ++++++++++++++++++++++++
 docs/source/framework/core/datetime.rst |   7 ++
 tests/HostTests/include/DateTimeData.h  |  18 +--
 tests/HostTests/modules/DateTime.cpp    |  21 +++-
 tests/HostTests/tools/datetime-test.py  |   5 +-
 10 files changed, 384 insertions(+), 40 deletions(-)
 create mode 100644 Sming/Core/ZonedTime.cpp
 create mode 100644 Sming/Core/ZonedTime.h

diff --git a/Sming/Core/DateTime.cpp b/Sming/Core/DateTime.cpp
index 4840e84841..bb1b6fde73 100644
--- a/Sming/Core/DateTime.cpp
+++ b/Sming/Core/DateTime.cpp
@@ -81,6 +81,40 @@ bool isLeapCentury(uint16_t year)
 
 } // namespace
 
+DateTime::ZoneInfo::Tag DateTime::ZoneInfo::Tag::fromString(const char* s)
+{
+	return s ? fromString(s, strlen(s)) : Tag{};
+}
+
+DateTime::ZoneInfo::Tag DateTime::ZoneInfo::Tag::fromString(const char* s, size_t len)
+{
+	Tag tag{};
+	if(s && len) {
+		len = std::min(maxSize, len);
+		memcpy(tag.value, s, len);
+		tag.value[len] = '\0';
+	}
+	return tag;
+}
+
+String DateTime::ZoneInfo::getOffsetString(char sep) const
+{
+	String s;
+	auto mins = offsetMins;
+	if(mins < 0) {
+		s += '-';
+		mins = -mins;
+	} else {
+		s += '+';
+	}
+	s.concat(mins / MINS_PER_HOUR, DEC, 2);
+	if(sep) {
+		s += sep;
+	}
+	s.concat(mins % MINS_PER_HOUR, DEC, 2);
+	return s;
+}
+
 bool DateTime::isLeapYear(uint16_t year)
 {
 	return year % 4 == 0 && isLeapCentury(year);
@@ -206,7 +240,7 @@ bool DateTime::fromHttpDate(const String& httpDate)
 	return true;
 }
 
-bool DateTime::fromISO8601(const String& datetime)
+bool DateTime::fromISO8601(const String& datetime, ZoneInfo* zone)
 {
 	auto ptr = datetime.c_str();
 	bool notDigit{false};
@@ -275,15 +309,44 @@ bool DateTime::fromISO8601(const String& datetime)
 			}
 		}
 	}
-	if(haveTime && notDigit) {
-		return false;
+
+	int16_t offsetMins{0};
+
+	if(haveTime) {
+		if(notDigit) {
+			return false;
+		}
+		int sign = 0;
+		if(skip('-')) {
+			sign = -1;
+		} else if(skip('+')) {
+			sign = 1;
+		} else {
+			skip('Z');
+		}
+		if(sign) {
+			auto hour = parseNumber(2);
+			skip(':');
+			auto min = parseNumber(2);
+			offsetMins = sign * (int(hour) * MINS_PER_HOUR + int(min));
+		}
 	}
 
 	if(*ptr != '\0') {
 		return false;
 	}
 
-	calcDayOfYear();
+	if(zone) {
+		*zone = ZoneInfo{.offsetMins = offsetMins};
+	}
+
+	if(offsetMins == 0 || zone) {
+		// No offset to apply
+		calcDayOfYear();
+	} else {
+		// Full recalculation required to correctly apply offset
+		setTime(toUnixTime() - int(offsetMins) * int(SECS_PER_MIN));
+	}
 
 	return true;
 }
@@ -308,9 +371,9 @@ String DateTime::toFullDateTimeString() const
 	return format(_F("%x %T"));
 }
 
-String DateTime::toISO8601() const
+String DateTime::toISO8601(const ZoneInfo* zone) const
 {
-	return format(_F("%FT%TZ"));
+	return format(zone ? _F("%FT%T%:z") : _F("%FT%TZ"), zone);
 }
 
 String DateTime::toHTTPDate() const
@@ -392,7 +455,7 @@ time_t DateTime::toUnixTime(int sec, int min, int hour, int day, uint8_t month,
 	return seconds;
 }
 
-String DateTime::format(const char* sFormat) const
+String DateTime::format(const char* sFormat, const ZoneInfo* zone) const
 {
 	if(sFormat == nullptr) {
 		return nullptr;
@@ -414,7 +477,24 @@ String DateTime::format(const char* sFormat) const
 		}
 
 		c = *sFormat++;
+		char timesep{'\0'};
+		if(c == ':') {
+			timesep = c;
+			c = *sFormat++;
+		}
 		switch(c) {
+		// Timezone offset from UTC with or without ':' separator
+		case 'z':
+			if(zone) {
+				sReturn += zone->getOffsetString(timesep);
+			}
+			break;
+		// Timezone tag
+		case 'Z':
+			if(zone) {
+				sReturn += zone->tag;
+			}
+			break;
 		// Year (not implemented: EY, Oy, Ey, EC, G, g)
 		case 'Y': // Full year as a decimal number, e.g. 2018
 			sReturn += Year;
diff --git a/Sming/Core/DateTime.h b/Sming/Core/DateTime.h
index cfc89bb952..e94aabb961 100644
--- a/Sming/Core/DateTime.h
+++ b/Sming/Core/DateTime.h
@@ -136,6 +136,49 @@ inline constexpr unsigned elapsedSecsThisWeek(time_t time)
 class DateTime
 {
 public:
+	/**
+	 * @brief Basic information required when displaying or handling local times.
+	 */
+	struct ZoneInfo {
+		/**
+		 * @brief Type for timezone abbreviation such as "GMT", "EEST"
+		 */
+		struct Tag {
+			static constexpr size_t maxSize = 5;
+			char value[maxSize + 1];
+
+			/**
+			 * @name String will be truncated if required and always NUL terminated.
+			 * @{
+			 */
+			static Tag fromString(const char* s);
+			static Tag fromString(const char* s, size_t len);
+			/** @} */
+
+			operator const char*() const
+			{
+				return value;
+			}
+		};
+
+		Tag tag{};			   ///< Abbreviation such as "GMT", "EEST" shown after time
+		int16_t offsetMins{0}; ///< Offset from UTC in minutes
+		bool isDst{false};	 ///< True if daylight savings is in effect
+
+		/**
+		 * @brief Get the offset in seconds so it can be added/subtracted directly from a time_t value
+		 */
+		int offsetSecs() const
+		{
+			return int(offsetMins) * SECS_PER_MIN;
+		}
+
+		/**
+		 * @brief Return offset in ISO8601 string format, e.g. +11:00
+		 */
+		String getOffsetString(char sep) const;
+	};
+
 	/** @brief  Instantiate an uninitialised date and time object
 	 */
 	DateTime()
@@ -193,6 +236,7 @@ class DateTime
 
 	/** @brief  Parse an ISO8601 date/time string
 	 *  @param  datetime Date and optional time in ISO8601 format, e.g. "1994-11-06", "1994-11-06T08:49:37". Separators are optional.
+	 *  @param  zone If provided, returns offset component of time
 	 *  @retval bool True on success
 	 *  @see See https://en.wikipedia.org/wiki/ISO_8601
 	 *
@@ -211,8 +255,18 @@ class DateTime
 	 * 	Thh:mm or Thhmm
 	 * 	Thh.hhh
 	 * 	Thh
+	 *
+	 * Times with an offset:
+	 *
+	 *  <time>Z
+	 * 	<time>±hh:mm
+	 * 	<time>±hhmm
+	 * 	<time>±hh
+	 *
+	 * If `zone` parameter is provided then the offset will be stored there.
+	 * Otherwise, the offset is subtracted from the time to produce UTC.
 	 */
-	bool fromISO8601(const String& datetime);
+	bool fromISO8601(const String& datetime, ZoneInfo* zone = nullptr);
 
 	/** @brief  Check if time date object is initialised
 	 *  @retval True if object has no value. False if initialised.
@@ -242,9 +296,12 @@ class DateTime
 	String toFullDateTimeString() const;
 
 	/** @brief  Get human readable date and time
-	 *  @retval String Date and time in format YYYY-MM-DDThh:mm:ssZ
+	 *  @param  zone Optional timezone information
+	 *  @retval String Date and time
+	 *  @see    See `fromISO8601()` for string formats
+	 *  @note   If `zone` isn't specified then UTC is assumed and timezone indicator 'Z' will be appended
 	 */
-	String toISO8601() const;
+	String toISO8601(const ZoneInfo* zone = nullptr) const;
 
 	/** @brief  Get human readable date and time
 	 *  @retval String Date and time in format DDD, DD MMM YYYY hh:mm:ss GMT
@@ -256,7 +313,6 @@ class DateTime
 	 */
 	void addMilliseconds(long add);
 
-	// functions to convert to and from time components (hrs, secs, days, years etc) to time_t
 	/** @brief  Convert from Unix time to individual time components
 	 *  @param  timep Unix time date value to convert
 	 *  @param  psec Pointer to integer to hold resulting seconds
@@ -276,8 +332,6 @@ class DateTime
 	static void fromUnixTime(time_t timep, uint8_t* psec, uint8_t* pmin, uint8_t* phour, uint8_t* pday, uint8_t* pwday,
 							 uint8_t* pmonth, uint16_t* pyear);
 
-	// functions to convert to and from time components (hrs, secs, days, years etc) to time_t
-
 	/** @brief  Convert from individual time components to Unix time
 	 *  @param  sec Seconds
 	 *  @param  min Minutes
@@ -294,6 +348,7 @@ class DateTime
 
 	/** @brief  Create string formatted with time and date placeholders
 	 *  @param  formatString String including date and time formatting
+	 *  @param  zone Optional timezone information
 	 *  @retval String Formatted string
 	 *  @note   Uses strftime style formatting, e.g. format("Today is %a, %d %b %Y") returns "Today is Mon, 10 Dec 2018"
 	 *  @note   Localisation may be implemented in libsming at compile time by setting LOCALE, e.g. LOCALE=LOCALE_DE_DE
@@ -333,20 +388,30 @@ class DateTime
 	 *  | %%X   | Locale preferred time representation | * |
 	 *  | %%y   | Year as a decimal number without a century (range 00 to 99) |  |
 	 *  | %%Y   | Year as a decimal number (range 1970 to ...) |  |
+	 *  | %%z   | Timezone offset in ±HHMM format (1) |  |
+	 *  | %%:z  | Timezone offset in ±HH:MM format (1) |  |
+	 *  | %%Z   | Timezone tag (1) |  |
 	 *  | %%    | Percent sign |  |
+	 *
+	 * (1) If zone is not provided then the %z, %:z and %Z placeholders produce empty text
 	 */
-	String format(const char* formatString) const;
+	String format(const char* formatString, const ZoneInfo* zone = nullptr) const;
 
 	/** @brief  Create string formatted with time and date placeholders
 	 *  @param  formatString String including date and time formatting
+	 *  @param  zone Optional zone information containing additional format information
 	 *  @retval String Formatted string
-	 *  @note see format(const char*) for parameter details
+	 *  @see 	See `format(const char*, const ZoneInfo*)` for parameter details
 	 */
-	String format(const String& formatString) const
+	String format(const String& formatString, const ZoneInfo* zone = nullptr) const
 	{
-		return format(formatString.c_str());
+		return format(formatString.c_str(), zone);
 	}
 
+	/**
+	 * @name Utility functions
+	 * @{
+	 */
 	static bool isLeapYear(uint16_t year);
 	static uint8_t getMonthDays(uint8_t month, uint16_t year);
 	static String getLocaleDayName(uint8_t day);
@@ -354,6 +419,7 @@ class DateTime
 	static String getIsoDayName(uint8_t day);
 	static String getIsoMonthName(uint8_t month);
 	static uint16_t getDaysInYear(uint16_t year);
+	/** @} */
 
 private:
 	// Helper methods
diff --git a/Sming/Core/SystemClock.cpp b/Sming/Core/SystemClock.cpp
index a59b24cec2..2baee4abaa 100644
--- a/Sming/Core/SystemClock.cpp
+++ b/Sming/Core/SystemClock.cpp
@@ -9,6 +9,7 @@
  ****/
 
 #include "SystemClock.h"
+#include "ZonedTime.h"
 #include <Platform/RTC.h>
 #include <debug_progmem.h>
 
@@ -19,7 +20,7 @@ time_t SystemClockClass::now(TimeZone timeType) const
 	uint32_t systemTime = RTC.getRtcSeconds();
 
 	if(timeType == eTZ_Local) {
-		systemTime += timeZoneOffsetSecs;
+		systemTime += zoneInfo.offsetSecs();
 	}
 
 	return systemTime;
@@ -28,7 +29,7 @@ time_t SystemClockClass::now(TimeZone timeType) const
 bool SystemClockClass::setTime(time_t time, TimeZone timeType)
 {
 	if(timeType == eTZ_Local) {
-		time -= timeZoneOffsetSecs;
+		time -= zoneInfo.offsetSecs();
 	}
 
 	timeSet = RTC.setRtcSeconds(time);
@@ -40,12 +41,13 @@ bool SystemClockClass::setTime(time_t time, TimeZone timeType)
 
 String SystemClockClass::getSystemTimeString(TimeZone timeType) const
 {
-	DateTime dt(now(timeType));
-	return dt.toFullDateTimeString();
-}
+	time_t systemTime = RTC.getRtcSeconds();
 
-bool SystemClockClass::setTimeZoneOffset(int seconds)
-{
-	timeZoneOffsetSecs = seconds;
-	return true;
+	if(timeType == eTZ_UTC) {
+		DateTime dt(systemTime);
+		return dt.toFullDateTimeString();
+	}
+
+	ZonedTime time(systemTime, zoneInfo);
+	return time.toString();
 }
diff --git a/Sming/Core/SystemClock.h b/Sming/Core/SystemClock.h
index e2f4a732b5..3d1a070daf 100644
--- a/Sming/Core/SystemClock.h
+++ b/Sming/Core/SystemClock.h
@@ -63,7 +63,7 @@ class SystemClockClass
      */
 	void setTimeZoneOffset(int seconds)
 	{
-		timeZoneOffsetSecs = seconds / SECS_PER_MIN;
+		zoneInfo.offsetMins = seconds / SECS_PER_MIN;
 	}
 
 	/** @brief Set the local time zone offset in hours
@@ -77,12 +77,28 @@ class SystemClockClass
 		setTimeZoneOffset(localTimezoneOffset * SECS_PER_HOUR);
 	}
 
+	/** @brief Set current time zone information
+     *  @param zoneInfo Contains offset plus string  formatting information
+	 */
+	void setTimeZone(const DateTime::ZoneInfo& zoneInfo)
+	{
+		this->zoneInfo = zoneInfo;
+	}
+
+	/** @brief Get current time zone information
+     *  @retval ZoneInfo
+	 */
+	const DateTime::ZoneInfo& getTimeZone() const
+	{
+		return zoneInfo;
+	}
+
 	/** @brief Get the current time zone offset
 	 *  @retval int Offset in seconds from UTC
 	 */
 	int getTimeZoneOffset() const
 	{
-		return timeZoneOffsetSecs;
+		return zoneInfo.offsetSecs();
 	}
 
 	/** @brief Determine if `setTime()` has been called yet
@@ -93,8 +109,17 @@ class SystemClockClass
 		return timeSet;
 	}
 
+	/**
+	 * @brief Set/clear a callback which is invoked whenever local time is queried.
+	 * @note This callback is *only* invoked when `now()` is called.
+	 */
+	void onCheckTimeZoneOffset(CheckTimeZoneOffset callback)
+	{
+		checkTimeZoneOffset = callback;
+	}
+
 private:
-	int timeZoneOffsetSecs = 0;
+	DateTime::ZoneInfo zoneInfo;
 	bool timeSet = false;
 };
 
diff --git a/Sming/Core/ZonedTime.cpp b/Sming/Core/ZonedTime.cpp
new file mode 100644
index 0000000000..e96d6b0928
--- /dev/null
+++ b/Sming/Core/ZonedTime.cpp
@@ -0,0 +1,3 @@
+#include "ZonedTime.h"
+
+CString ZonedTime::defaultFormat;
diff --git a/Sming/Core/ZonedTime.h b/Sming/Core/ZonedTime.h
new file mode 100644
index 0000000000..3647a991e4
--- /dev/null
+++ b/Sming/Core/ZonedTime.h
@@ -0,0 +1,139 @@
+#pragma once
+
+#include "DateTime.h"
+#include <Data/CString.h>
+
+/**
+ * @brief A timestamp representing a UTC 'point in time' associated with a specific timezone
+ *
+ * Derivation of the correct `DateTime::ZoneInfo` value is complex and typically managed by
+ * an external library, such as :library:`Timezone`.
+ *
+ * The implicit `time_t` value for a ZonedTime is the corresponding UTC time.
+ * All comparisons or arithmetic operations will therefore operate using this value.
+ * This means two ZonedTime objects will be considered equal if their corresponding UTC
+ * timestamps match, even if they derive from different timezones.
+ *
+ * To obtain the `time_t` value with local offset applied, use `local()`:
+ *
+ * 		DateTime dtLocal(zonedTime.local());
+ *
+ * There is no support in this class for manipulating time or zone information directly.
+ * This should be done using the raw `time_t` value and a new ZonedTime object constructed.
+ */
+class ZonedTime
+{
+public:
+	/**
+	 * @brief Construct a zoned time
+	 * @param utc
+	 * @param zoneInfo
+	 */
+	ZonedTime(time_t utc, const DateTime::ZoneInfo& zoneInfo) : utc(utc), zoneInfo(zoneInfo)
+	{
+	}
+
+	/**
+	 * @brief Construct a time in the UTC zone
+	 */
+	explicit ZonedTime(time_t utc = 0) : utc(utc), zoneInfo{{"UTC"}}
+	{
+	}
+
+	ZonedTime(const ZonedTime& other) = default;
+
+	operator time_t() const
+	{
+		return utc;
+	}
+
+	/**
+	 * @brief Get a ZonedTime instance translated into the UTC zone
+	 * This does nothing more than replace the contained zone information with UTC,
+	 * which has tag "UTC", no offset and no daylight savings.
+	 */
+	ZonedTime toUtc() const
+	{
+		return ZonedTime(utc);
+	}
+
+	/**
+	 * @brief Obtain the `time_t` value adjusted for local time
+	 */
+	time_t local() const
+	{
+		return utc + zoneInfo.offsetSecs();
+	}
+
+	/**
+	 * @brief Get instance zone information
+	 */
+	const DateTime::ZoneInfo& getZoneInfo() const
+	{
+		return zoneInfo;
+	}
+
+	const DateTime::ZoneInfo::Tag& tag() const
+	{
+		return zoneInfo.tag;
+	}
+
+	int16_t offsetMins() const
+	{
+		return zoneInfo.offsetMins;
+	}
+
+	int offsetSecs() const
+	{
+		return zoneInfo.offsetSecs();
+	}
+
+	bool isDst() const
+	{
+		return zoneInfo.isDst;
+	}
+
+	/**
+	 * @name Format time using `DateTime`
+	 * @see   See `DateTime::format()` for details
+	 * @{
+	 */
+	String format(const char* formatString) const
+	{
+		DateTime dt(local());
+		return dt.format(formatString, &zoneInfo);
+	}
+
+	String format(const String& formatString) const
+	{
+		return format(formatString.c_str());
+	}
+
+	String toISO8601() const
+	{
+		DateTime dt(local());
+		return dt.toISO8601(&zoneInfo);
+	}
+	/** @} */
+
+	/**
+	 * @brief Obtain default formatted string
+	 */
+	String toString() const
+	{
+		return format(defaultFormat ? defaultFormat.c_str() : _F("%a %d %b %Y %T %Z"));
+	}
+
+	/**
+	 * @brief Set the default format for dispaying zoned times
+	 */
+	static void setDefaultFormat(const String& format)
+	{
+		defaultFormat = format;
+	}
+
+private:
+	static CString defaultFormat;
+	time_t utc;
+	DateTime::ZoneInfo zoneInfo;
+};
diff --git a/docs/source/framework/core/datetime.rst b/docs/source/framework/core/datetime.rst
index f6a10661a2..4291cefee3 100644
--- a/docs/source/framework/core/datetime.rst
+++ b/docs/source/framework/core/datetime.rst
@@ -7,6 +7,13 @@ DateTime class
 .. doxygenclass:: DateTime
    :members:
 
+ZonedTime class
+---------------
+
+.. doxygenclass:: ZonedTime
+   :members:
+
+
 System Clock
 ------------
 
diff --git a/tests/HostTests/include/DateTimeData.h b/tests/HostTests/include/DateTimeData.h
index 7f1a2d03ce..d77707ac5d 100644
--- a/tests/HostTests/include/DateTimeData.h
+++ b/tests/HostTests/include/DateTimeData.h
@@ -2,7 +2,7 @@
 /*
     * DateTime test data
     *
-    * File generated Thu Apr 11 14:30:36 2024
+    * File generated Mon May 27 21:53:09 2024
     *
     */
 
@@ -43,15 +43,17 @@ struct TestDate {
   XX(DT_19, "2106-02-07T06:28:15", "2106-02-07T06:28:15Z", 0xffffffffLL, 0) \
   XX(DT_20, "2106-02-07T06:28:15", "2106-02-07T06:28:15Z", 0xffffffffLL, 0) \
   XX(DT_21, "2106-02-07T06:28:16", "2106-02-07T06:28:16Z", 0x100000000LL, 0) \
-  XX(DT_22, "1901-12-13T20:45:52", "1901-12-13T20:45:52Z", -0x80000000LL, 0)
+  XX(DT_22, "1901-12-13T20:45:52", "1901-12-13T20:45:52Z", -0x80000000LL, 0) \
+  XX(DT_23, "2024-05-22T06:28:15+07:30", "2024-05-21T22:58:15Z", 0x664d2707LL, 0) \
+  XX(DT_24, "2024-05-22T06:28:15-23:45", "2024-05-23T06:13:15Z", 0x664ede7bLL, 0)
 
 #define VALID_ISO_TIME_MAP(XX) \
-  XX(DT_23, "T18:47:12.123", "18:47:12.123", 0x10830LL, 123) \
-  XX(DT_24, "T18:47:12.123456", "18:47:12.123", 0x10830LL, 123) \
-  XX(DT_25, "T12:34:12", "12:34:12", 0xb0c4LL, 0) \
-  XX(DT_26, "T23:59:59", "23:59:59", 0x1517fLL, 0) \
-  XX(DT_27, "12:34", "12:34:00", 0xb0b8LL, 0) \
-  XX(DT_28, "T2359", "23:59:00", 0x15144LL, 0)
+  XX(DT_25, "T18:47:12.123", "18:47:12.123", 0x10830LL, 123) \
+  XX(DT_26, "T18:47:12.123456", "18:47:12.123", 0x10830LL, 123) \
+  XX(DT_27, "T12:34:12", "12:34:12", 0xb0c4LL, 0) \
+  XX(DT_28, "T23:59:59", "23:59:59", 0x1517fLL, 0) \
+  XX(DT_29, "12:34", "12:34:00", 0xb0b8LL, 0) \
+  XX(DT_30, "T2359", "23:59:00", 0x15144LL, 0)
 
 
 #define XX(tag, str, str2, ...) \
diff --git a/tests/HostTests/modules/DateTime.cpp b/tests/HostTests/modules/DateTime.cpp
index 544789b0c9..2fdd960ac4 100644
--- a/tests/HostTests/modules/DateTime.cpp
+++ b/tests/HostTests/modules/DateTime.cpp
@@ -37,8 +37,25 @@ class DateTimeTest : public TestGroup
 			}
 		}
 
-		// dt = testDate;
-		// debug_d("\"%s\"", dt.toFullDateTimeString().c_str());
+		TEST_CASE("fromISO8601 with offset")
+		{
+			auto check = [&](const String& strWithOffset, const String& strUtc, time_t time, int offsetMins) {
+				DateTime dt;
+				// Without zone, offset is applied to time
+				CHECK(dt.fromISO8601(strWithOffset));
+				REQUIRE_EQ(dt.toUnixTime(), time);
+				REQUIRE_EQ(dt.toISO8601(), strUtc);
+				// With zone, get local time plus offset
+				DateTime::ZoneInfo zoneInfo;
+				CHECK(dt.fromISO8601(strWithOffset, &zoneInfo));
+				REQUIRE_EQ(dt.toUnixTime(), time + (offsetMins * 60));
+				REQUIRE_EQ(zoneInfo.offsetMins, offsetMins);
+				REQUIRE_EQ(dt.toISO8601(&zoneInfo), strWithOffset);
+			};
+
+			check(F("2024-05-22T06:28:15+12:45"), F("2024-05-21T17:43:15Z"), 1716313395, 12 * 60 + 45);
+			check(F("2024-05-22T06:28:15-23:45"), F("2024-05-23T06:13:15Z"), 1716444795, -(23 * 60 + 45));
+		}
 
 		TEST_CASE("setTime")
 		{
diff --git a/tests/HostTests/tools/datetime-test.py b/tests/HostTests/tools/datetime-test.py
index 77c0af69c3..66e4d430d5 100644
--- a/tests/HostTests/tools/datetime-test.py
+++ b/tests/HostTests/tools/datetime-test.py
@@ -59,9 +59,10 @@ def parse_iso_datetime(value: str | tuple | int) -> Record:
     else:
         s = value
         dt = datetime.datetime.fromisoformat(value)
+        dt = dt.astimezone(datetime.timezone.utc)
     if dt.tzinfo is None:
         dt = dt.replace(tzinfo=datetime.timezone.utc)
-    iso_string = dt.isoformat().removesuffix('+00:00') + 'Z'
+    iso_string = dt.isoformat().replace('+00:00', 'Z')
     return Record(s, iso_string, dt.timestamp(), dt.microsecond // 1000)
 
 
@@ -135,6 +136,8 @@ def parse_iso_time(value: str) -> Record:
             "2106-02-07T06:28:15",
             0x100000000,
             -0x80000000,
+            "2024-05-22T06:28:15+07:30",
+            "2024-05-22T06:28:15-23:45",
         ]),
     'VALID_ISO_TIME': (
         parse_iso_time,

From 7fc4c2b465a74a3ac43be8504d4de028dfefed82 Mon Sep 17 00:00:00 2001
From: mikee47 <mike@silyhouse.net>
Date: Mon, 27 May 2024 22:01:18 +0100
Subject: [PATCH 4/8] Ensure DateTime:: fromHttpDate(), fromISO8601() and
 setTime() set weekday correctly

Requires full conversion to/from time_t.
So add static versions of these methods which can also be used where broken-down time isn't required.
Any failure in parsing means the DateTime value is left unchanged, rather than in an indeterminate state.
---
 Sming/Core/DateTime.cpp | 111 +++++++++++++++++++++++++---------------
 Sming/Core/DateTime.h   |  52 +++++++++++--------
 2 files changed, 101 insertions(+), 62 deletions(-)

diff --git a/Sming/Core/DateTime.cpp b/Sming/Core/DateTime.cpp
index bb1b6fde73..688d1ee3ed 100644
--- a/Sming/Core/DateTime.cpp
+++ b/Sming/Core/DateTime.cpp
@@ -155,7 +155,7 @@ bool DateTime::isNull() const
 		   Milliseconds == 0;
 }
 
-bool DateTime::fromHttpDate(const String& httpDate)
+bool DateTime::fromHttpDate(const String& httpDate, time_t& time)
 {
 	auto ptr = httpDate.c_str();
 
@@ -170,7 +170,9 @@ bool DateTime::fromHttpDate(const String& httpDate)
 
 	skipWhitespace();
 	if(!isdigit(*ptr)) {
-		if(!matchName(ptr, DayofWeek, isoDayNames, 7)) {
+		// Don't actually use this value
+		uint8_t weekday;
+		if(!matchName(ptr, weekday, isoDayNames, 7)) {
 			return false; // Invalid day of week
 		}
 		if(ptr[0] == ',') {
@@ -185,7 +187,7 @@ bool DateTime::fromHttpDate(const String& httpDate)
 	}
 
 	skipWhitespace();
-	Day = parseNumber();
+	uint8_t day = parseNumber();
 	if(*ptr != '-' && !isspace(*ptr)) {
 		return false;
 	}
@@ -194,7 +196,8 @@ bool DateTime::fromHttpDate(const String& httpDate)
 	// Should we check DayOfWeek against calculation from date?
 	// We could just ignore the DOW...
 	skipWhitespace();
-	if(!matchName(ptr, Month, isoMonthNames, 12)) {
+	uint8_t month;
+	if(!matchName(ptr, month, isoMonthNames, 12)) {
 		return false; // Invalid month
 	}
 	if(*ptr != '-') {
@@ -209,38 +212,47 @@ bool DateTime::fromHttpDate(const String& httpDate)
 	++ptr;
 
 	skipWhitespace();
-	Year = parseNumber();
+	uint16_t year = parseNumber();
 	if(*ptr++ != ' ') {
 		return false;
 	}
-	if(Year < 70) {
-		Year += 2000;
-	} else if(Year < 100) {
-		Year += 1900;
+	if(year < 70) {
+		year += 2000;
+	} else if(year < 100) {
+		year += 1900;
 	}
 
 	skipWhitespace();
-	Hour = parseNumber();
+	uint8_t hour = parseNumber();
 	if(*ptr++ != ':') {
 		return false;
 	}
-	Minute = parseNumber();
+	uint8_t minute = parseNumber();
 	if(*ptr++ != ':') {
 		return false;
 	}
-	Second = parseNumber();
+	uint8_t second = parseNumber();
 
 	if(*ptr != '\0' && strcmp(ptr, " GMT") != 0) {
 		return false;
 	}
 
-	Milliseconds = 0;
-	calcDayOfYear();
+	time = toUnixTime(second, minute, hour, day, month, year);
 
 	return true;
 }
 
-bool DateTime::fromISO8601(const String& datetime, ZoneInfo* zone)
+bool DateTime::fromHttpDate(const String& httpDate)
+{
+	time_t time;
+	if(!fromHttpDate(httpDate, time)) {
+		return false;
+	}
+	setTime(time);
+	return true;
+}
+
+bool DateTime::fromISO8601(const String& datetime, time_t& time, uint16_t& milliseconds, int16_t& offsetMins)
 {
 	auto ptr = datetime.c_str();
 	bool notDigit{false};
@@ -271,19 +283,22 @@ bool DateTime::fromISO8601(const String& datetime, ZoneInfo* zone)
 
 	bool haveTime = skip('T') || ptr[2] == ':';
 
+	uint16_t year;
+	uint8_t month;
+	uint8_t day;
 	if(haveTime) {
-		Year = 1970;
-		Month = dtJanuary;
-		Day = 1;
+		year = 1970;
+		month = dtJanuary;
+		day = 1;
 	} else {
-		Year = parseNumber(4);
+		year = parseNumber(4);
 		skip('-');
-		Month = parseNumber(2) - 1;
+		month = parseNumber(2) - 1;
 		skip('-');
 		if(*ptr == '\0' || isspace(*ptr)) {
-			Day = 1;
+			day = 1;
 		} else {
-			Day = parseNumber(2);
+			day = parseNumber(2);
 		}
 		if(notDigit) {
 			return false;
@@ -291,18 +306,19 @@ bool DateTime::fromISO8601(const String& datetime, ZoneInfo* zone)
 		haveTime = skip('T');
 	}
 
-	Hour = parseNumber(2);
+	uint8_t hour = parseNumber(2);
 	skip(':');
-	Minute = parseNumber(2);
+	uint8_t minute = parseNumber(2);
 	skip(':');
-	Milliseconds = 0;
+	milliseconds = 0;
+	uint8_t second;
 	if(*ptr == '\0') {
-		Second = 0;
+		second = 0;
 	} else {
-		Second = parseNumber(2);
+		second = parseNumber(2);
 		if(*ptr == '.') {
 			++ptr;
-			Milliseconds = parseNumber(3);
+			milliseconds = parseNumber(3);
 			// Discard any microsecond digits
 			while(isdigit(*ptr)) {
 				++ptr;
@@ -310,7 +326,7 @@ bool DateTime::fromISO8601(const String& datetime, ZoneInfo* zone)
 		}
 	}
 
-	int16_t offsetMins{0};
+	offsetMins = 0;
 
 	if(haveTime) {
 		if(notDigit) {
@@ -336,18 +352,29 @@ bool DateTime::fromISO8601(const String& datetime, ZoneInfo* zone)
 		return false;
 	}
 
-	if(zone) {
-		*zone = ZoneInfo{.offsetMins = offsetMins};
+	// Full recalculation ensures offset applied and day of week calculated
+	time = toUnixTime(second, minute, hour, day, month, year);
+
+	return true;
+}
+
+bool DateTime::fromISO8601(const String& datetime, ZoneInfo* zone)
+{
+	time_t time;
+	uint16_t milliseconds;
+	int16_t offsetMins;
+	if(!fromISO8601(datetime, time, milliseconds, offsetMins)) {
+		return false;
 	}
 
-	if(offsetMins == 0 || zone) {
-		// No offset to apply
-		calcDayOfYear();
+	if(zone) {
+		zone->offsetMins = offsetMins;
 	} else {
-		// Full recalculation required to correctly apply offset
-		setTime(toUnixTime() - int(offsetMins) * int(SECS_PER_MIN));
+		time -= int(offsetMins) * SECS_PER_MIN;
 	}
 
+	setTime(time);
+	Milliseconds = milliseconds;
 	return true;
 }
 
@@ -518,14 +545,14 @@ String DateTime::format(const char* sFormat, const ZoneInfo* zone) const
 			break;
 		// Week (not implemented: OU, OW, OV)
 		case 'U': // Week of the year as a decimal number (Sunday is the first day of the week) [00..53]
-			sReturn.concat(calcWeek(0), DEC, 2);
+			sReturn.concat(getWeekOfYear(dtSunday), DEC, 2);
 			break;
 		case 'V': // ISO 8601 week number (01-53)
 			// !@todo Calculation of ISO 8601 week number is crude and frankly wrong but does anyone care?
-			sReturn.concat(calcWeek(1) + 1, DEC, 2);
+			sReturn.concat(getWeekOfYear(dtMonday) + 1, DEC, 2);
 			break;
 		case 'W': // Week of the year as a decimal number (Monday is the first day of the week) [00..53]
-			sReturn.concat(calcWeek(1), DEC, 2);
+			sReturn.concat(getWeekOfYear(dtMonday), DEC, 2);
 			break;
 		case 'x': // Locale preferred date format
 			sReturn += format(_F(LOCALE_DATE));
@@ -620,14 +647,14 @@ void DateTime::calcDayOfYear()
 	DayofYear = prevMonthDays + Day;
 }
 
-uint8_t DateTime::calcWeek(uint8_t firstDay) const
+uint8_t DateTime::getWeekOfYear(dtDays_t firstDay) const
 {
 	int16_t startOfWeek = DayofYear - DayofWeek + firstDay;
-	int8_t firstDayofWeek = startOfWeek % 7;
+	int8_t firstDayofWeek = startOfWeek % DAYS_PER_WEEK;
 	if(firstDayofWeek < 0) {
 		firstDayofWeek += 7;
 	}
-	return (startOfWeek + 7 - firstDayofWeek) / 7;
+	return (startOfWeek + 7 - firstDayofWeek) / DAYS_PER_WEEK;
 }
 
 String DateTime::getLocaleDayName(uint8_t day)
diff --git a/Sming/Core/DateTime.h b/Sming/Core/DateTime.h
index e94aabb961..f7c7c10260 100644
--- a/Sming/Core/DateTime.h
+++ b/Sming/Core/DateTime.h
@@ -117,6 +117,8 @@ inline constexpr unsigned elapsedSecsThisWeek(time_t time)
 }
 
 /** @brief  Date and time class
+ *
+ * This class contains a 'broken-down' date and time into its individual year, month, etc. components.
  *
  * Date and time functions mostly work with Unix time, the quantity of seconds since 00:00:00 1970-01-01.
  * There is no support for leap seconds which are added (and in theory, removed) occasionally to compensate for earth rotation variation.
@@ -216,31 +218,43 @@ class DateTime
 	 */
 	void setTime(uint8_t sec, uint8_t min, uint8_t hour, uint8_t day, uint8_t month, uint16_t year)
 	{
-		Second = sec;
-		Minute = min;
-		Hour = hour;
-		Day = day;
-		Month = month;
-		Year = year;
-		Milliseconds = 0;
-		calcDayOfYear();
+		setTime(toUnixTime(sec, min, hour, day, month, year));
 	}
 
 	/** @brief  Parse a HTTP full date and set time and date
 	 *  @param  httpDate HTTP full date in RFC 1123 format, e.g. Sun, 06 Nov 1994 08:49:37 GMT
 	 *  @retval bool True on success
+	 *  @see    See `fromHttpDate(const String& time_t&)`
+	 */
+	bool fromHttpDate(const String& httpDate);
+
+	/** @brief  Parse a HTTP full date string and return the time_t value
+	 *  @param  httpDate HTTP full date in RFC 1123 format, e.g. Sun, 06 Nov 1994 08:49:37 GMT
+	 *  @param  time On success, contains the decoded value
+	 *  @retval bool True on success
 	 *  @note   Also supports obsolete RFC 850 date format, e.g. Sunday, 06-Nov-94 08:49:37 GMT where 2 digit year represents range 1970-2069
 	 *  @note   GMT suffix is optional and is always assumed / ignored
 	 */
-	bool fromHttpDate(const String& httpDate);
+	static bool fromHttpDate(const String& httpDate, time_t& time);
 
 	/** @brief  Parse an ISO8601 date/time string
 	 *  @param  datetime Date and optional time in ISO8601 format, e.g. "1994-11-06", "1994-11-06T08:49:37". Separators are optional.
-	 *  @param  zone If provided, returns offset component of time
+	 *  @param  zone If provided, on success the `offsetMins` field will contain the time offset (0 for GMT) and the
+	 * 			decoded DateTime will be 'local'. If zone is null then the decoded datetime will be UTC.
+	 *  @retval bool True on success. On failure, value of DateTime is unchanged.
+	 *  @see    See `fromISO8601(const String&, time_t&, uint16_t&, int16_t&)`
+	 */
+	bool fromISO8601(const String& datetime, ZoneInfo* zone = nullptr);
+
+	/** @brief  Parse an ISO8601 date/time string and return discrete components
+	 *  @param  datetime Date and optional time in ISO8601 format, e.g. "1994-11-06", "1994-11-06T08:49:37". Separators are optional.
+	 *  @param  time The time_t component
+	 *  @param  milliseconds Additional milliseconds value
+	 *  @param  offsetMins Any offset specified in the time
 	 *  @retval bool True on success
-	 *  @see See https://en.wikipedia.org/wiki/ISO_8601
+	 *  @see    See https://en.wikipedia.org/wiki/ISO_8601
 	 *
-	 * `Basic format` doesn't include separators, whereas `Extended format` does.
+	 * `Basic format` doesn't include separators, whereas `Extended format` does. Both are supported.
 	 *
 	 * Acceptable date formats:
 	 *
@@ -262,11 +276,8 @@ class DateTime
 	 * 	<time>±hh:mm
 	 * 	<time>±hhmm
 	 * 	<time>±hh
-	 *
-	 * If `zone` parameter is provided then the offset will be stored there.
-	 * Otherwise, the offset is subtracted from the time to produce UTC.
 	 */
-	bool fromISO8601(const String& datetime, ZoneInfo* zone = nullptr);
+	static bool fromISO8601(const String& datetime, time_t& time, uint16_t& milliseconds, int16_t& offsetMins);
 
 	/** @brief  Check if time date object is initialised
 	 *  @retval True if object has no value. False if initialised.
@@ -310,6 +321,7 @@ class DateTime
 
 	/** @brief  Add time to date time object
 	 *  @param  add Quantity of milliseconds to add to object
+	 *  @note   This operation is computationally expensive, requiring conversion to and from time_t.
 	 */
 	void addMilliseconds(long add);
 
@@ -322,10 +334,8 @@ class DateTime
 	 *  @param  pwday Pointer to integer to hold resulting day of week
 	 *  @param  pmonth Pointer to integer to hold resulting month
 	 *  @param  pyear Pointer to integer to hold resulting year
-	 *  @note   This is a more compact version of the C library localtime function
 	 *  @note   Pass the Unix timedate value and pointers to existing integers. The integers are updated with the converted values
 	 *  @note   This static function  may be used without instantiating a DateTime object, e.g. DateTime::convertFromUnixTime(...);
-	 *  @note   32-bit Unix time has year 2036 issue.
 	 *  @note   Unix time does not account for leap seconds.
 	 *  @note   All of the return values are optional, specify nullptr if not required
 	 */
@@ -423,8 +433,10 @@ class DateTime
 
 private:
 	// Helper methods
-	void calcDayOfYear();					  // Calculate day of year
-	uint8_t calcWeek(uint8_t firstDay) const; // Calculate week number based on firstDay of week
+	void calcDayOfYear();
+
+	// Calculate week number based on firstDay of week
+	uint8_t getWeekOfYear(dtDays_t firstDay) const;
 
 public:
 	uint16_t Year = 0;		   ///< Full Year number

From 6a219c294a80f14590b3ce426b7c9d3c75b16847 Mon Sep 17 00:00:00 2001
From: mikee47 <mike@silyhouse.net>
Date: Mon, 27 May 2024 11:20:08 +0100
Subject: [PATCH 5/8] `DT_DATE_SEPARATOR` no longer used

---
 Sming/Core/SystemClock.h | 1 -
 1 file changed, 1 deletion(-)

diff --git a/Sming/Core/SystemClock.h b/Sming/Core/SystemClock.h
index 3d1a070daf..5efc9f0c92 100644
--- a/Sming/Core/SystemClock.h
+++ b/Sming/Core/SystemClock.h
@@ -53,7 +53,6 @@ class SystemClockClass
 	/** @brief  Get current time as a string
      *  @param  timeType Time zone to present time as, i.e. return local or UTC time
      *  @retval String Current time in format: `dd.mm.yy hh:mm:ss`
-     *  @note   Date separator may be changed by adding `#define DT_DATE_SEPARATOR "/"` to source code
      */
 	String getSystemTimeString(TimeZone timeType = eTZ_Local) const;
 

From 3c84c9f82431c3682ef6a49e820befe34dba0507 Mon Sep 17 00:00:00 2001
From: mikee47 <mike@silyhouse.net>
Date: Fri, 31 May 2024 21:04:23 +0100
Subject: [PATCH 6/8] Add `onCheckTimeZoneOffset()` method to SystemClock

---
 Sming/Core/SystemClock.cpp |  8 +++++++-
 Sming/Core/SystemClock.h   | 18 ++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/Sming/Core/SystemClock.cpp b/Sming/Core/SystemClock.cpp
index 2baee4abaa..855d2dfee3 100644
--- a/Sming/Core/SystemClock.cpp
+++ b/Sming/Core/SystemClock.cpp
@@ -20,6 +20,9 @@ time_t SystemClockClass::now(TimeZone timeType) const
 	uint32_t systemTime = RTC.getRtcSeconds();
 
 	if(timeType == eTZ_Local) {
+		if(checkTimeZoneOffset) {
+			checkTimeZoneOffset(systemTime);
+		}
 		systemTime += zoneInfo.offsetSecs();
 	}
 
@@ -41,7 +44,10 @@ bool SystemClockClass::setTime(time_t time, TimeZone timeType)
 
 String SystemClockClass::getSystemTimeString(TimeZone timeType) const
 {
-	time_t systemTime = RTC.getRtcSeconds();
+	time_t systemTime = now(eTZ_UTC);
+	if(checkTimeZoneOffset) {
+		checkTimeZoneOffset(systemTime);
+	}
 
 	if(timeType == eTZ_UTC) {
 		DateTime dt(systemTime);
diff --git a/Sming/Core/SystemClock.h b/Sming/Core/SystemClock.h
index 5efc9f0c92..d13466ef02 100644
--- a/Sming/Core/SystemClock.h
+++ b/Sming/Core/SystemClock.h
@@ -34,6 +34,23 @@ enum TimeZone {
 class SystemClockClass
 {
 public:
+	/**
+	 * @brief Optional callback which can be used to automate handling of timezone changes
+	 * @param systemTime The current system time
+	 *
+	 * Although the timezone offset only changes twice a year, when it does change the
+	 * corresponding offset should be updated promptly.
+	 *
+	 * `setTimeZoneOffset()` is typically called when updating via real-time clock chip or NTP,
+	 * but this can still leave a considerable gap during which the local time will be wrong.
+	 *
+	 * The callback should check the provided systemTime and determine if the time zone offset
+	 * requires changing and, if so, call `setTimeZoneOffset()`.
+	 * One way to make this efficient is to cache the `time_t` value for the *next* change,
+	 * rather than attemtpting to compute the offset on every call.
+	 */
+	using CheckTimeZoneOffset = Delegate<void(time_t systemTime)>;
+
 	/** @brief  Get the current date and time
      *  @param  timeType Time zone to use (UTC / local)
      *  @retval DateTime Current date and time
@@ -118,6 +135,7 @@ class SystemClockClass
 	}
 
 private:
+	CheckTimeZoneOffset checkTimeZoneOffset;
 	DateTime::ZoneInfo zoneInfo;
 	bool timeSet = false;
 };

From e46df744ce2dbb989d98df94df4b3f85f7b681d6 Mon Sep 17 00:00:00 2001
From: mikee47 <mike@silyhouse.net>
Date: Tue, 23 Apr 2024 20:30:47 +0100
Subject: [PATCH 7/8] SystemClock_NTP sample: Timezone code relates to system,
 move out of class

---
 samples/SystemClock_NTP/app/NtpClientDemo.cpp | 66 +++++++++++--------
 samples/SystemClock_NTP/app/application.cpp   |  3 +-
 .../SystemClock_NTP/include/NtpClientDemo.h   | 16 ++---
 3 files changed, 43 insertions(+), 42 deletions(-)

diff --git a/samples/SystemClock_NTP/app/NtpClientDemo.cpp b/samples/SystemClock_NTP/app/NtpClientDemo.cpp
index 7ed6c6ec1d..8fb026c185 100644
--- a/samples/SystemClock_NTP/app/NtpClientDemo.cpp
+++ b/samples/SystemClock_NTP/app/NtpClientDemo.cpp
@@ -1,9 +1,44 @@
 #include <NtpClientDemo.h>
+#include <SystemClock.h>
+#include <HardwareSerial.h>
 #include <SolarCalculator.h>
 
-const TimeChangeRule NtpClientDemo::dstStart = {"BST", Last, Sun, Mar, 1, 60};
-const TimeChangeRule NtpClientDemo::stdStart = {"GMT", Last, Sun, Oct, 2, 0};
-Timezone NtpClientDemo::tz(dstStart, stdStart);
+namespace
+{
+/*
+ * For handling local/UTC time conversions
+ * This is for the UK, amend as required
+ */
+const TimeChangeRule dstStart{"BST", Last, Sun, Mar, 1, 60};
+const TimeChangeRule stdStart{"GMT", Last, Sun, Oct, 2, 0};
+Timezone tz(dstStart, stdStart);
+
+/*
+ * We use the y/m/d from local time for sunrise/sunset calculations, and the solar calculator
+ * returns the time from midnight in UTC for that day. We therefore need to adjust this
+ * to account for timezone and daylight savings.
+ */
+time_t getNextSunriseSet(bool isSunrise)
+{
+	auto timeNow = SystemClock.now(eTZ_Local);
+	DateTime dt(timeNow);
+	dt.Hour = 0;
+	dt.Minute = 0;
+	dt.Second = 0;
+	SolarCalculator calc;
+	int offset_secs = SECS_PER_MIN * calc.sunRiseSet(isSunrise, dt.Year, dt.Month + 1, dt.Day);
+
+	time_t t = tz.toLocal(dt + offset_secs);
+
+	// If time has already passed, then make it tomorrow
+	if(t < timeNow) {
+		t = tz.toLocal(dt + offset_secs + SECS_PER_DAY);
+	}
+
+	return t;
+}
+
+} // namespace
 
 void NtpClientDemo::ntpResult(NtpClient& client, time_t ntpTime)
 {
@@ -30,28 +65,3 @@ void NtpClientDemo::ntpResult(NtpClient& client, time_t ntpTime)
 	Serial << _F("Next sunrise at ") << sunrise.toShortTimeString() << _F(", sunset at ") << sunset.toShortTimeString()
 		   << endl;
 }
-
-/*
- * We use the y/m/d from local time for sunrise/sunset calculations, and the solar calculator
- * returns the time from midnight in UTC for that day. We therefore need to adjust this
- * to account for timezone and daylight savings.
- */
-time_t NtpClientDemo::getNextSunriseSet(bool isSunrise)
-{
-	auto timeNow = SystemClock.now(eTZ_Local);
-	DateTime dt(timeNow);
-	dt.Hour = 0;
-	dt.Minute = 0;
-	dt.Second = 0;
-	SolarCalculator calc;
-	int offset_secs = SECS_PER_MIN * calc.sunRiseSet(isSunrise, dt.Year, dt.Month + 1, dt.Day);
-
-	time_t t = tz.toLocal(dt + offset_secs);
-
-	// If time has already passed, then make it tomorrow
-	if(t < timeNow) {
-		t = tz.toLocal(dt + offset_secs + SECS_PER_DAY);
-	}
-
-	return t;
-}
diff --git a/samples/SystemClock_NTP/app/application.cpp b/samples/SystemClock_NTP/app/application.cpp
index ecd3121802..077fa39682 100644
--- a/samples/SystemClock_NTP/app/application.cpp
+++ b/samples/SystemClock_NTP/app/application.cpp
@@ -9,8 +9,7 @@
 
 namespace
 {
-#pragma GCC diagnostic ignored "-Wunused-function"
-void onNtpReceive(NtpClient& client, time_t timestamp);
+[[maybe_unused]] void onNtpReceive(NtpClient& client, time_t timestamp);
 
 SimpleTimer printTimer;
 
diff --git a/samples/SystemClock_NTP/include/NtpClientDemo.h b/samples/SystemClock_NTP/include/NtpClientDemo.h
index 9f6dbbfed0..037bf76cb2 100644
--- a/samples/SystemClock_NTP/include/NtpClientDemo.h
+++ b/samples/SystemClock_NTP/include/NtpClientDemo.h
@@ -1,26 +1,18 @@
 #pragma once
 
-#include <SmingCore.h>
+#include <Network/NtpClient.h>
+#include <SystemClock.h>
 #include <Timezone.h>
 
 class NtpClientDemo
 {
 public:
-	NtpClientDemo() : ntpcp(nullptr, 30, NtpTimeResultDelegate(&NtpClientDemo::ntpResult, this))
+	NtpClientDemo() : client(nullptr, 30, NtpTimeResultDelegate(&NtpClientDemo::ntpResult, this))
 	{
 	}
 
 	void ntpResult(NtpClient& client, time_t ntpTime);
 
-	time_t getNextSunriseSet(bool isSunrise);
-
 private:
-	NtpClient ntpcp;
-	/*
-	 * For handling local/UTC time conversions
-	 * This is for the UK, amend as required
-	 */
-	static const TimeChangeRule dstStart;
-	static const TimeChangeRule stdStart;
-	static Timezone tz;
+	NtpClient client;
 };

From 612366411cd8cbadf53a20221e0a64413d73d3f0 Mon Sep 17 00:00:00 2001
From: mikee47 <mike@silyhouse.net>
Date: Tue, 28 May 2024 11:45:10 +0100
Subject: [PATCH 8/8] Update `SystemClock_NTP` sample and Timezone library

Add to Windows CI builds
---
 Sming/Libraries/Timezone                      |  2 +-
 Tools/ci/build.cmd                            |  2 +
 samples/SystemClock_NTP/app/NtpClientDemo.cpp | 89 +++++++++++++++----
 .../SystemClock_NTP/include/NtpClientDemo.h   |  2 -
 4 files changed, 74 insertions(+), 21 deletions(-)

diff --git a/Sming/Libraries/Timezone b/Sming/Libraries/Timezone
index 9af032b5b5..92ceeaff61 160000
--- a/Sming/Libraries/Timezone
+++ b/Sming/Libraries/Timezone
@@ -1 +1 @@
-Subproject commit 9af032b5b5820434f94c715a39da787b7ab8301b
+Subproject commit 92ceeaff611f5df80143b5d3092e0a50f4859108
diff --git a/Tools/ci/build.cmd b/Tools/ci/build.cmd
index 8744dc8fbf..3f4d7185ae 100644
--- a/Tools/ci/build.cmd
+++ b/Tools/ci/build.cmd
@@ -36,6 +36,8 @@ REM HostTests must build for all architectures
 REM Building Bear SSL library causes memory issues with CI builds in Windows, so avoid for now
 %MAKE_PARALLEL% -C "%SMING_PROJECTS_DIR%/tests/HostTests" %TEST_FLAGS% || goto :error
 
+%MAKE_PARALLEL% -C "%SMING_HOME%" SystemClock_NTP || goto :error
+
 REM Start Arch-specific tests
 cd /d %SMING_HOME%
 call Arch\%SMING_ARCH%\Tools\ci\build.run.cmd || goto :error
diff --git a/samples/SystemClock_NTP/app/NtpClientDemo.cpp b/samples/SystemClock_NTP/app/NtpClientDemo.cpp
index 8fb026c185..aefa2bf30f 100644
--- a/samples/SystemClock_NTP/app/NtpClientDemo.cpp
+++ b/samples/SystemClock_NTP/app/NtpClientDemo.cpp
@@ -2,40 +2,71 @@
 #include <SystemClock.h>
 #include <HardwareSerial.h>
 #include <SolarCalculator.h>
+#include <Timezone.h>
+#include <tzdata.h>
 
-namespace
+namespace MyZone
 {
+using namespace TZ;
 /*
  * For handling local/UTC time conversions
  * This is for the UK, amend as required
  */
-const TimeChangeRule dstStart{"BST", Last, Sun, Mar, 1, 60};
-const TimeChangeRule stdStart{"GMT", Last, Sun, Oct, 2, 0};
-Timezone tz(dstStart, stdStart);
+const Rule dstStart{"BST", Last, Sun, Mar, 1, 60};
+const Rule stdStart{"GMT", Last, Sun, Oct, 2, 0};
+
+// Posix rule string
+DEFINE_FSTR_LOCAL(tzstr, "GMT0BST,M3.5.0/1,M10.5.0")
+
+// Sunrise/sunset requires co-ordinates
+SolarRef solarRef = {51.4769, 0.0005}; // Greenwich, London
+
+} // namespace MyZone
+
+namespace
+{
+// We can initialise timezone in various ways
+// Timezone tz = Timezone::fromPosix(MyZone::tzstr);
+// Timezone tz(MyZone::dstStart, MyZone::stdStart);
+Timezone tz = TZ::Europe::London();
 
 /*
  * We use the y/m/d from local time for sunrise/sunset calculations, and the solar calculator
  * returns the time from midnight in UTC for that day. We therefore need to adjust this
  * to account for timezone and daylight savings.
  */
-time_t getNextSunriseSet(bool isSunrise)
+ZonedTime getNextSunriseSet(bool isSunrise)
 {
-	auto timeNow = SystemClock.now(eTZ_Local);
-	DateTime dt(timeNow);
+	auto utcNow = SystemClock.now(eTZ_UTC);
+	DateTime dt(tz.toLocal(utcNow));
 	dt.Hour = 0;
 	dt.Minute = 0;
 	dt.Second = 0;
-	SolarCalculator calc;
+	SolarCalculator calc(MyZone::solarRef);
 	int offset_secs = SECS_PER_MIN * calc.sunRiseSet(isSunrise, dt.Year, dt.Month + 1, dt.Day);
 
-	time_t t = tz.toLocal(dt + offset_secs);
+	time_t utcNext = dt + offset_secs;
 
 	// If time has already passed, then make it tomorrow
-	if(t < timeNow) {
-		t = tz.toLocal(dt + offset_secs + SECS_PER_DAY);
+	if(utcNext < utcNow) {
+		utcNext += SECS_PER_DAY;
 	}
 
-	return t;
+	return tz.makeZoned(utcNext);
+}
+
+void checkTimeZoneOffset(time_t systemTime)
+{
+	static ZonedTime nextChange{TZ::invalidTime};
+
+	if(nextChange == TZ::invalidTime) {
+		nextChange = tz.makeZoned(systemTime);
+	} else if(systemTime < nextChange) {
+		return;
+	}
+
+	SystemClock.setTimeZone(nextChange.getZoneInfo());
+	nextChange = tz.getNextChange(systemTime);
 }
 
 } // namespace
@@ -46,9 +77,10 @@ void NtpClientDemo::ntpResult(NtpClient& client, time_t ntpTime)
 	 * Update the system clock and calculate the correct time offset,
 	 * accounting for time zone and daylight savings.
 	 */
-	auto localTime = tz.toLocal(ntpTime);
 	SystemClock.setTime(ntpTime, eTZ_UTC);
-	SystemClock.setTimeZoneOffset(localTime - ntpTime);
+
+	// Now we've set the clock, we can determine the initial active timezone and maintain the offset
+	SystemClock.onCheckTimeZoneOffset([this](time_t systemTime) { checkTimeZoneOffset(systemTime); });
 
 	/*
 	 * Display the new time
@@ -60,8 +92,29 @@ void NtpClientDemo::ntpResult(NtpClient& client, time_t ntpTime)
 	/*
 	 * Display times of next sunrise and sunset
 	 */
-	DateTime sunrise = getNextSunriseSet(true);
-	DateTime sunset = getNextSunriseSet(false);
-	Serial << _F("Next sunrise at ") << sunrise.toShortTimeString() << _F(", sunset at ") << sunset.toShortTimeString()
-		   << endl;
+	ZonedTime sunrise = getNextSunriseSet(true);
+	ZonedTime sunset = getNextSunriseSet(false);
+	Serial << _F("Next sunrise ") << sunrise.toString() << _F(", sunset ") << sunset.toString() << endl;
+
+	/*
+	 * Display points at which daylight savings changes.
+	 */
+	if(!tz.hasDaylightSavings()) {
+		Serial << _F("Selected timezone has no daylight savings.") << endl;
+		return;
+	}
+
+	DateTime dt(ntpTime);
+
+	Serial << _F("In ") << dt.Year << _F(", daylight savings:") << endl;
+	ZonedTime dst = tz.getTransition(dt.Year, true);
+	ZonedTime transition = tz.makeZoned(dst, true);
+	Serial << _F("  Starts: ") << transition.toString() << " (" << dst.toUtc().toString() << ")" << endl;
+
+	ZonedTime std = tz.getTransition(dt.Year, false);
+	transition = tz.makeZoned(std, true);
+	Serial << _F("    Ends: ") << transition.toString() << " (" << std.toUtc().toString() << ")" << endl;
+
+	ZonedTime nextChange = tz.getNextChange(ntpTime);
+	Serial << _F("Next change to ") << nextChange.tag() << _F(" on ") << nextChange.toUtc().toString() << endl;
 }
diff --git a/samples/SystemClock_NTP/include/NtpClientDemo.h b/samples/SystemClock_NTP/include/NtpClientDemo.h
index 037bf76cb2..d12c502963 100644
--- a/samples/SystemClock_NTP/include/NtpClientDemo.h
+++ b/samples/SystemClock_NTP/include/NtpClientDemo.h
@@ -1,8 +1,6 @@
 #pragma once
 
 #include <Network/NtpClient.h>
-#include <SystemClock.h>
-#include <Timezone.h>
 
 class NtpClientDemo
 {