The following examples show some pitfalls of Date/Time arithmetic with regard to DST transitions and months having different numbers of days.
Example #1 DateTimeImmutable::add/sub add intervals which cover elapsed time
Adding PT24H over a DST transition will appear to add 23/25 hours (for most timezones).
<?php
$dt = new DateTimeImmutable("2015-11-01 00:00:00", new DateTimeZone("America/New_York"));
echo "Start: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt = $dt->add(new DateInterval("PT3H"));
echo "End: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
?>The above example will output:
Start: 2015年11月01日 00:00:00 -04:00 End: 2015年11月01日 02:00:00 -05:00
Example #2 DateTimeImmutable::modify and strtotime increment or decrement individual component values
Adding +24 hours over a DST transition will add exactly 24 hours as seen in the date/time string (unless the start or end time is on a transition point).
<?php
$dt = new DateTimeImmutable("2015-11-01 00:00:00", new DateTimeZone("America/New_York"));
echo "Start: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt = $dt->modify("+24 hours");
echo "End: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
?>The above example will output:
Start: 2015年11月01日 00:00:00 -04:00 End: 2015年11月02日 00:00:00 -05:00
Example #3 Adding or subtracting times can over- or underflow dates
Like where January 31st + 1 month will result in March 2nd (leap year) or 3rd (normal year).
<?php
echo "Normal year:\n"; // February has 28 days
$dt = new DateTimeImmutable("2015-01-31 00:00:00", new DateTimeZone("America/New_York"));
echo "Start: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt = $dt->modify("+1 month");
echo "End: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
echo "Leap year:\n"; // February has 29 days
$dt = new DateTimeImmutable("2016-01-31 00:00:00", new DateTimeZone("America/New_York"));
echo "Start: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt = $dt->modify("+1 month");
echo "End: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
?>The above example will output:
Normal year: Start: 2015年01月31日 00:00:00 -05:00 End: 2015年03月03日 00:00:00 -05:00 Leap year: Start: 2016年01月31日 00:00:00 -05:00 End: 2016年03月02日 00:00:00 -05:00
To get the last day of the next month (i.e. to prevent the overflow),
the last day of format is available.
<?php
echo "Normal year:\n"; // February has 28 days
$dt = new DateTimeImmutable("2015-01-31 00:00:00", new DateTimeZone("America/New_York"));
echo "Start: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt = $dt->modify("last day of next month");
echo "End: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
echo "Leap year:\n"; // February has 29 days
$dt = new DateTimeImmutable("2016-01-31 00:00:00", new DateTimeZone("America/New_York"));
echo "Start: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt = $dt->modify("last day of next month");
echo "End: ", $dt->format("Y-m-d H:i:s P"), PHP_EOL;
?>The above example will output:
Normal year: Start: 2015年01月31日 00:00:00 -05:00 End: 2015年02月28日 00:00:00 -05:00 Leap year: Start: 2016年01月31日 00:00:00 -05:00 End: 2016年02月29日 00:00:00 -05:00
Be careful when subtracting a duration crossing a DST change, it can output a resulting date AFTER your base date.
// tested in 8.3.18, 7.4.33
// DST timezone shift is applied properly when setting time explicitely
echo (new DateTime('2025-03-30'))->setTimeZone(new DateTimeZone('Europe/Paris'))->setTime(1,59,0)->format('c').PHP_EOL;
echo (new DateTime('2025-03-30'))->setTimeZone(new DateTimeZone('Europe/Paris'))->setTime(2,0,0)->format('c').PHP_EOL;
echo (new DateTime('2025-03-30'))->setTimeZone(new DateTimeZone('Europe/Paris'))->setTime(2,1,0)->format('c').PHP_EOL;
// 2025年03月30日T01:59:00+01:00 < correct
// 2025年03月30日T03:00:00+02:00 < correct
// 2025年03月30日T03:01:00+02:00 < correct
echo PHP_EOL;
// DST timezone shift is applied properly when addind a duration
$startDateTime = (new DateTime('2025-03-30'))->setTimeZone(new DateTimeZone('Europe/Paris'))->setTime(1,58,0);
echo $startDateTime->format('c').PHP_EOL;
$endDatetime = clone($startDateTime)->add(DateInterval::createFromDateString('4 minutes'));
echo $endDatetime->format('c').PHP_EOL;
// 2025年03月30日T01:58:00+01:00 < correct
// 2025年03月30日T03:02:00+02:00 < correct
echo PHP_EOL;
// DST timezone shift is applied improperly when subtracting a duration
$startDateTime = (new DateTime('2025-03-30'))->setTimeZone(new DateTimeZone('Europe/Paris'))->setTime(3,2,0);
echo $startDateTime->format('c').PHP_EOL;
$endDatetime = clone($startDateTime)->sub(DateInterval::createFromDateString('4 minutes'));
echo $endDatetime->format('c').PHP_EOL;
// 2025年03月30日T03:02:00+02:00 < correct
// 2025年03月30日T03:58:00+02:00 < incorrect !!!
echo PHP_EOL;
// DST timezone shift is still applied improperly when adding a negative duration
$startDateTime = (new DateTime('2025-03-30'))->setTimeZone(new DateTimeZone('Europe/Paris'))->setTime(3,2,0);
echo $startDateTime->format('c').PHP_EOL;
$endDatetime = clone($startDateTime)->add(DateInterval::createFromDateString('-4 minutes'));
echo $endDatetime->format('c').PHP_EOL;
// 2025年03月30日T03:02:00+02:00 < correct
// 2025年03月30日T03:58:00+02:00 < incorrect !!!