Add onCheckTimeZoneOffset() method to SystemClock, update demo

Sming/Core/SystemClock.cpp

@@ -19,6 +19,9 @@ time_t SystemClockClass::now(TimeZone timeType) const
 	uint32_t systemTime = RTC.getRtcSeconds();
 
 	if(timeType == eTZ_Local) {
+		if(checkTimeZoneOffset) {
+			checkTimeZoneOffset(systemTime);
+		}
 		systemTime += timeZoneOffsetSecs;
 	}
 

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
@@ -88,7 +105,17 @@ 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:
+	CheckTimeZoneOffset checkTimeZoneOffset;
 	int timeZoneOffsetSecs = 0;
 	bool timeSet = false;
 };

samples/SystemClock_NTP/app/NtpClientDemo.cpp

@@ -38,6 +38,21 @@ time_t getNextSunriseSet(bool isSunrise)
 	return t;
 }
 
+void checkTimeZoneOffset(time_t systemTime)
+{
+	static time_t nextChange;
+	static const TimeChangeRule* rule;
+
+	if(!rule) {
+		tz.toLocal(systemTime, &rule);
+	} else if(systemTime < nextChange) {
+		return;
+	}
+
+	SystemClock.setTimeZoneOffset(rule->offset * SECS_PER_MIN);
+	nextChange = tz.getNextChange(systemTime, &rule);
+}
+
 } // namespace
 
 void NtpClientDemo::ntpResult(NtpClient& client, time_t ntpTime)
@@ -46,9 +61,10 @@ 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(checkTimeZoneOffset);
 
 	/*
 	 * Display the new time
@@ -64,4 +80,21 @@ void NtpClientDemo::ntpResult(NtpClient& client, time_t ntpTime)
 	DateTime sunset = getNextSunriseSet(false);
 	Serial << _F("Next sunrise at ") << sunrise.toShortTimeString() << _F(", sunset at ") << sunset.toShortTimeString()
 		   << endl;
+
+	/*
+	 * Display points at which daylight savings changes.
+	 */
+	DateTime dt(ntpTime);
+
+	auto& dstRule = tz.getRule(true);
+	DateTime dstDateTime = dstRule(dt.Year);
+	Serial << dstRule.tag << _F(" starts ") << dstDateTime.toHTTPDate() << endl;
+
+	auto& stdRule = tz.getRule(false);
+	DateTime stdDateTime = stdRule(dt.Year);
+	Serial << stdRule.tag << _F(" starts ") << stdDateTime.toHTTPDate() << endl;
+
+	const TimeChangeRule* rule;
+	time_t nextChange = tz.getNextChange(dt, &rule);
+	Serial << _F("Next change to ") << rule->tag << _F(" is ") << DateTime(nextChange).toHTTPDate() << endl;
 }