How to convert server date and time to the local date and time taking into account daylight saving time

‘Many countries, or even parts of countries, adopt daylight saving time (DST, also known as “Summer Time”) during part of the year. This typically involves advancing clocks by an hour near the start of spring and adjusting back in autumn (“spring” forward, “fall” back).’ (see Wikipedia)

Now I will demonstrate how we can convert a server date and time to the corresponding date and time in another timezone in Europe, taking Romania as example.

‘All countries in Europe except Iceland observe DST, and most change on the same date and time, starting on the last Sunday in March and ending on the last Sunday in October.’
‘In the West European (UTC), Central European (CET, UTC+1), and East European (UTC+2) time zones the change is simultaneous: on both dates the clocks are changed everywhere at 01:00 UTC, i.e. from local times of 01:00/02:00/03:00 to 02:00/03:00/04:00 in March, and vice versa in October.’ (see Wikipedia)

Romania is a country in the Eastern Europe, which falls under the East European Time (EET) also known as UTC+2 time zone and uses Eastern European Summer Time (UTC+3) as a summer daylight saving time.

The next function will be used to find out the offset in hours from UTC (GMT) of a local date and time of Romania or another region that switches between DST and standard time on the same date and time as Romania (is the local date and time in DST or in standard time?)

function local_offset_from_utc($timestamp, $standard_offset)
//$timestamp - the timestamp of the server date and time we want to convert to local date and time
//$standard_offset - the offset from UTC of the local date and time when observing the standard time; in daylight saving time the offset is $standard_offset + 1
{
	$year = gmdate("Y", $timestamp); //get the UTC year of the given date and time
//get the date to 'spring forward'
	$mar_31_utc_ts = gmmktime(0, 0, 0, 4, 0, $year); //the UTC timestamp of March 31, the last day of March, equivalent to gmmktime(0, 0, 0, 3, 31, $year)
	$last_sun_mar = 31 - gmdate("w", $mar_31_utc_ts); //the day of the month under which falls the last Sunday in March
	$spring_ts = gmmktime(1, 0, 0, 3, $last_sun_mar, $year); //the timestamp of the last Sunday in March 01:00 UTC
//get the date to 'fall back'
	$oct_31_utc_ts = gmmktime(0, 0, 0, 11, 0, $year); //the UTC timestamp of October 31, the last day of October, equivalent to gmmktime(0, 0, 0, 10, 31, $year)
	$last_sun_oct = 31 - gmdate("w", $oct_31_utc_ts); //the day of the month under which falls the last Sunday in October
	$fall_ts = gmmktime(1, 0, 0, 10, $last_sun_oct, $year); //the timestamp of the last Sunday in October, 01:00 UTC
	if($timestamp >= $spring_ts && $timestamp < $fall_ts) //daylight saving time
	{
		return $standard_offset + 1;
	}
//standard time
	return $standard_offset;
}

If the server switches between DST and standard time simultaneously with your timezone, you can simply use date("I") to find out if your local date and time is in daylight saving time or standard time:

if(date("I", $timestamp) == 1)
{
	$offset = $standard_offset + 1;
}
else
{
	$offset = $standard_offset;
}

Be careful with that, because a sever don't always switch between DST and standard time at the same hour as the timezone it has set, for example my server switches from DST to standard time at 03:00 UTC+3, which becomes 02:00 UTC+2, when it should go from 04:00 UTC+3 to 03:00 UTC+2). So I better use the function above that works not caring if or when the timezone of the server switches between DST and standard time.

If an offset of a timezone from UTC is negative, then the UTC date and time is greater than the corresponding date and time in the considered timezone; if the offset is positive then the UTC date and time is lower than the corresponding date and time in the considered timezone. If we consider t being the date and time in a given timezone, utc being the corresponding UTC date and time and offset being the offset of the given timezone from UTC, then we have:

t = utc + offset
utc = t - offset

So if we consider t1 being a server date and time and o1 being the offset of the timezone of the server from UTC (in hours) and t2 being the local date and time and o2 being the offset of the timezone of the server from UTC (in hours) we have:

utc = t1 - o1
t2 = utc + o2 = t1 - o1 + o2

In practice we only have a server date and time which we convert to timestamp ts1 based on which we find out the timestamp ts2 when the date and time in the timezone of the server equaled/will equal to the local date and time corresponding to ts1:

ts2 = ts1 - os1 + os2, where os1 = o1 * 60 *60 seconds and os2 = o2 * 60 *60 seconds

E.g. Let's consider the timezone of the server date and time is UTC-5 and Romania is in standard time (has an offset of 2 hours from UTC).

The UTC date and time that corresponds to the given timestamp equals to the server date and time that was/will be 5 hours later than the given timestamp:

ts_aux = ts1 - (-5 * 3600) = ts1 + 5 * 3600

The local date and time of Romania corresponding to the given timestamp equals to the UTC date and time that was/will be 2 hours later than the UTC date and time computed above:

ts2 = ts_aux + 2 * 3600 = ts1 + (5 + 2) * 3600 = ts1 + 7 * 3600

But this is NOT true if the server time and date that equals the needed local date and time falls under DST and the original server date falls under standard time or vice versa, which means that the offsets from UTC of the two server dates are different by 1 hour.

For example, if the server we considered before is in UTC-5 DST and goes back one hour on the first of November at 03:00:00 local server time and we want to see what Romania local date and time corresponds to the server date and time November 1st 01:00:00, then, applying the formula above, we will obtain a timstamp that corresponds to the server date and time November 1st 07:00:00 which should correspond to the local Romania time for the original server time, but the correct Romania date and time being November 1st 08:00:00. That is because, between November 1st 01:00:00 and November 1st 07:00:00, the server goes back one hour so that 03:00:00 DST becomes 02:00:00 standard time and 08:00:00 becomes 07:00:00, while the Romania time does not, Romania switching to DST one week earlier, so the correct formula in this case is:

ts2 = ts1 - os1 + os2 + 3600
ts2 = ts1 + (5 + 2 + 1) * 3600 = ts1 + 8 * 3600

Similarly, if the server switches from standard time to DST between the two dates, the formula becomes:

ts2 = ts1 - os1 + os2 - 3600
ts2 = ts1 + (5 + 2 - 1) * 3600 = ts1 + 6 * 3600

The next function is used to determine what the timestamp was/will be in the timezone of the server when the server date and time equaled/will be equal to the needed local date and time.

function get_timestamp_for_local($date = "", $standard_offset = 2)
{
	if(!empty($date))
	{
		$timestamp = strtotime($date);
	}
	else
	{
		$timestamp = time();
	}
	$os1 = date("Z", $timestamp);
	$os2 = local_offset_from_utc($timestamp, $standard_offset) * 3600;
	$ts = $timestamp - $os1 + $os2;
	//check the offset from UTC of the server date and time that equal the needed local date and time
	$os3 = date("Z", $ts);
	if($os3 == $os1) //the two server dates are both in DST or both in standard time
	{
		return $ts;
	}
	elseif($os3 > $os1) //the server date and time that equal the needed local date and time are in DST and the original server date and time are in standard time
	{
		return $ts - 3600;
	}
 	//the server date and time that equal the needed local date and time are in standard time and the original server date and time are in DST
	return $ts + 3600;
}

And finally we get the needed date and time:

function get_ro_date($date)
{
	return date("d.m.Y H:i:s", get_timestamp_for_local($date));
}

Next l will show how we can convert a server date and time to the corresponding date and time in another timezone in North America, taking the State of Washington as example.

'North America generally follows the same procedure, with each time zone switching at 02:00 LST (local standard time) to 03:00 LDT (local daylight time) on the second Sunday in March, and back from 02:00 LDT to 01:00 LST on the first Sunday in November since 2007.'
In the United States of America, 'the start of DST now occurs on the second Sunday in March and ends on the first Sunday in November.' (see Wikipedia)

The State of Washington falls under the Pacific Time Zone (PT), which is Pacific Standard Time (PST) or UTC-8 when observing standard time and Pacific Daylight Time (PDT) or UTC-7 during daylight saving time.

So we have to change the previous local_offset_from_utc function to find out the offset in hours from UTC (GMT) of a local date and time of Washington:

function local_offset_from_utc2($timestamp, $standard_offset, $spring_forward_hour, $fall_back_hour)
//$timestamp - the timestamp of the server date and time we want to convert to local date and time
//$standard_offset - the offset from UTC of the local date and time when observing the standard time; in daylight saving time the offset is $standard_offset + 1
//$spring_forward_hour - the hour of the local time when switching to DST
//$fall_back_hour - the hour of the local time when switching to standard time
{
//find out the year of the local date and time corresponding to the given server date and time, based on the corresponding UTC date and time
	$year = gmdate("Y", $timestamp); //UTC year of the given server date and time
	if($standard_offset < 0) //local timezone is behind UTC
	{
		//if the given sever date and time corresponds to January the 1st in UTC and the UTC hour is lower than the absolute value of the offset of the local time from UTC, then the corresponding local date and time falls in the year previous to the UTC year
		$month = gmdate("n", $timestamp);
		if($month == 1)
		{
			$day = gmdate("j", $timestamp);
			if($day == 1)
			{
				$hour = gmdate("G", $timestamp);
				if($hour + $standard_offset < 0)
				{
					$year -= 1;
				}
			}
		}
	}
	else //local timezone time is ahead of UTC
	{
		//if the given sever date and time corresponds to December 31 in UTC and the sum of UTC hour and the offset of the local time from UTC is greater than 24, then the corresponding local date and time falls in the year subsequent to the UTC year
		$month = gmdate("n", $timestamp);
		if($month == 12)
		{
			$day = gmdate("j", $timestamp);
			if($day == 31)
			{
				$hour = gmdate("G", $timestamp);
				if($hour + $standard_offset > 24)
				{
					$year += 1;
				}
			}
		}
	}
	//get the date to 'spring forward'
	$mar_1_utc_ts = gmmktime(0, 0, 0, 3, 1, $year); //the timestamp for the UTC time of March the 1st, 00:00:00
	$mar_1_day_of_week = gmdate("w", $mar_1_utc_ts);
	//compute the day of the month under which falls the second Sunday in March
	if($mar_1_day_of_week > 0) //the day of the week of the 1st of March is not Sunday
	{
		$second_sun_mar = 15 - $mar_1_day_of_week;
	}
	else
	{
		$second_sun_mar = 8;
	}
	$spring_local_ts = gmmktime($spring_forward_hour - $standard_offset, 0, 0, 3, $second_sun_mar, $year);
	//get the date to 'fall back'
	$nov_1_utc_ts = gmmktime(0, 0, 0, 11, 1, $year); //the timestamp for the UTC time of November the 1st, 00:00:00
	$nov_1_day_of_week = gmdate("w", $nov_1_utc_ts);
//compute the day of the month under which falls the first Sunday in November
	if($nov_1_day_of_week > 0) //the day of the week of the 1st of November is not Sunday
	{
		$first_sun_nov = 8 - $nov_1_day_of_week;
	}
	else
	{
		$first_sun_nov = 1;
	}
	$fall_local_ts = gmmktime($fall_back_hour - ($standard_offset + 1), 0, 0, 11, $first_sun_nov, $year);
	if($timestamp >= $spring_local_ts && $timestamp < $fall_local_ts) //daylight saving time
	{
		return $standard_offset + 1;
	}
	//standard time
	return $standard_offset;
}

The function get_timestamp_for_local remains the same, only we add 2 more paramters to it which will be needed when calling the function:

function get_timestamp_for_local2($date = "", $standard_offset = -8, $spring_forward_hour = 2, $fall_back_hour = 3)
{
	if(!empty($date))
	{
		$timestamp = strtotime($date);
	}
	else
	{
		$timestamp = time();
	}
	$os1 = date("Z", $timestamp);
	$os2 = local_offset_from_utc2($timestamp, $standard_offset, $spring_forward_hour, $fall_back_hour) * 3600;
	$ts = $timestamp - $os1 + $os2;
	//check the offset from UTC of the server date and time that equal the needed local date and time
	$os3 = date("Z", $ts);
	if($os3 == $os1) //the two dates are both in DST or both in standard time
	{
		return $ts;
	}
	elseif($os3 > $os1) //the server date and time that equal the needed local date and time are in DST and the original server date and time are in standard time
	{
		return $ts - 3600;
	}
	//the server date and time that equal the needed local date and time are in standard time and the original server date and time are in DST
	return $ts + 3600;
}

And now we can find out the date and time in the State of Washington that corresponds to the given server date and time:

function get_wa_date($date)
{
	return date("Y-m-d H:i:s", get_timestamp_for_local2($date));
}

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