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;
// 2025-03-30T01:59:00+01:00 < correct
// 2025-03-30T03:00:00+02:00 < correct
// 2025-03-30T03:01:00+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;
// 2025-03-30T01:58:00+01:00 < correct
// 2025-03-30T03:02:00+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;
// 2025-03-30T03:02:00+02:00 < correct
// 2025-03-30T03:58:00+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;
// 2025-03-30T03:02:00+02:00 < correct
// 2025-03-30T03:58:00+02:00 < incorrect !!!
