[PHP-NOTES] note 130190 added to datetime.examples-arithmetic

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 !!!
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Manual Page -- https://php.net/manual/en/datetime.examples-arithmetic.php
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