Description:
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I've seen many bugs that stem from how modifying dates, such as by strtotime or DateTime::modify, can cause unexpected results. Mostly due to DST transitions and how months have different numbers of days (looking at you, February). Such as bug #72470 just now. PHP tackles the problem in a fairly 'nix-standardized way that is not always obvious to a developer, and I think there should be documentation explaining how it works.

The three main topics are:

1. DateTime::add/sub add intervals which cover elapsed time, so adding PT24H over a DST transition will appear to add 23/25 hours (for most timezones).

Note that the below code should give that output but doesn't due to bug #72398.

<?php
$dt = new DateTime("2016-03-13 00:00:00", new DateTimeZone("America/New_York"));
echo "Start: ", $dt->format("Y-m-d H:i:s P"), "\n"; // 2016-03-13 00:00:00 -05:00
$dt->add(new DateInterval("PT24H"));
echo "End:   ", $dt->format("Y-m-d H:i:s P"), "\n"; // 2016-03-14 01:00:00 -04:00
?>

2. DateTime::modify and strtotime will increment or decrement individual component values, so 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 DateTime("2016-03-13 00:00:00", new DateTimeZone("America/New_York"));
echo "Start: ", $dt->format("Y-m-d H:i:s P"), "\n"; // 2016-03-13 00:00:00 -05:00
$dt->modify("+24 hours");
echo "End:   ", $dt->format("Y-m-d H:i:s P"), "\n"; // 2016-03-14 00:00:00 -04:00
?>

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). For that example I suggest also giving a solution to how one can "prevent" the overflow and only get the last day of the month.

<?php
echo "Normal year:\n"; // February has 28 days
$dt = new DateTime("2015-01-31 00:00:00", new DateTimeZone("America/New_York"));
echo "Start: ", $dt->format("Y-m-d H:i:s P"), "\n"; // 2016-01-31 00:00:00 -05:00
$dt->modify("+1 month");
echo "End:   ", $dt->format("Y-m-d H:i:s P"), "\n"; // 2016-03-03 00:00:00 -05:00

echo "Leap year:\n"; // February has 29 days
$dt = new DateTime("2016-01-31 00:00:00", new DateTimeZone("America/New_York"));
echo "Start: ", $dt->format("Y-m-d H:i:s P"), "\n"; // 2016-01-31 00:00:00 -05:00
$dt->modify("+1 month");
echo "End:   ", $dt->format("Y-m-d H:i:s P"), "\n"; // 2016-03-02 00:00:00 -05:00
?>