Examples
PHP Manual

Date/Time Arithmetic

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: 2015-11-01 00:00:00 -04:00
End:   2015-11-01 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: 2015-11-01 00:00:00 -04:00
End:   2015-11-02 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: 2015-01-31 00:00:00 -05:00
End:   2015-03-03 00:00:00 -05:00
Leap year:
Start: 2016-01-31 00:00:00 -05:00
End:   2016-03-02 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: 2015-01-31 00:00:00 -05:00
End:   2015-02-28 00:00:00 -05:00
Leap year:
Start: 2016-01-31 00:00:00 -05:00
End:   2016-02-29 00:00:00 -05:00