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;
?>

以上例程会输出：

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;
?>

以上例程会输出：

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;
?>

以上例程会输出：

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;
?>

以上例程会输出：

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

User Contributed Notes

info at mobger dot de 10-Jan-2021 08:13
There is a pitfall with the summertime in the timezone Europe/Berlin.

Beginning Summer-Timer
=======================

The hour between 2020-03-29 02:00:00 and 2020-03-29 03:00:00 is missing in the Timezone 'Europe/Berlin'.

Explanation to the code below
----------------------------------
The DateInterval of 20700 minutes point to 2020-03-29 03:00:00.
The DateInterval of 20701 minutes points to the not existing '2020-03-29 02:59:00', which is resolved to '2020-03-29 03:59:59' by rounding up.
The DateInterval-object of 20761 minutes calculate the existing '2020-03-29 01:59:00'. The hour between 2:00 - 3:00 does not exist in the euopean summertime for the date 2020-03-29. The missing hour will be additionally calculated in the substraction. This explain the result '2020-03-29 00:59:00'. It seems, that the addition of the DateInterval-object does not recalculate the missing hour for some (?) reasons.

<?php

        $test = [];
        $check = date_create_from_format('Y-m-d H:i:s', '2020-04-12 12:00:00', new DateTimeZone('Europe/Berlin'));
        for ($i = 0; $i < 30000; $i++) {
            $expectEaster = clone $check;
            $interval = new DateInterval('PT' . $i . 'M');
            $expectEaster->sub($interval);
            $dummy = clone $expectEaster;
            $expectEaster->add($interval);
            if (!in_array($expectEaster->format('Y-m-d H:i:s'), $test)) {
                $test[$i] = $expectEaster->format('Y-m-d H:i:s');
                echo($i . ' near summertime start ' . $expectEaster->format('Y-m-d H:i:s') . '[' . $dummy->format('Y-m-d H:i:s') . ']' . "\n");
            }
        }
        //        0 near summertime start 2020-04-12 12:00:00[2020-04-12 12:00:00]
        //    20701 near summertime start 2020-04-12 13:00:00[2020-03-29 03:59:00]
        //    20761 near summertime start 2020-04-12 11:00:00[2020-03-29 00:59:00]

?>

Ending Summer Timer
====================
The european summertime defines the existence of two hours with the notation 2020-10-25 02:00:00 - 2020-10-25 03:00:00 (a) and 2020-10-25 02:00:00 - 2020-10-25 03:00:00 (b).
It seems, that the dateTime-object handles the additional hour on an simple internal way. It seems, that you can't differ between this two hours (a) and (b) with the DateTime-object in a simple way.

<?php
        $check = date_create_from_format('Y-m-d H:i:s', '2020-10-25 06:00:00', new DateTimeZone('Europe/Berlin'));
        for ($i = 0; $i < 361; $i = $i + 60) {
            $expectEaster = clone $check;
            $interval = new DateInterval('PT' . $i . 'M');
            $expectEaster->sub($interval);
            $dummy = clone $expectEaster;
            $expectEaster->add($interval);
            $test[$i] = $expectEaster->format('Y-m-d H:i:s');
            echo($i . ' near summertime end ' . $expectEaster->format('Y-m-d H:i:s') .
                '[' . $dummy->format('Y-m-d H:i:s') . ' / ' . $dummy->getTimestamp() . ']' .
                "\n");
        }
//      0 near summertime end 2020-10-25 06:00:00[2020-10-25 06:00:00 / 1603602000]
//     60 near summertime end 2020-10-25 06:00:00[2020-10-25 05:00:00 / 1603598400]
//    120 near summertime end 2020-10-25 06:00:00[2020-10-25 04:00:00 / 1603594800]
//    180 near summertime end 2020-10-25 06:00:00[2020-10-25 03:00:00 / 1603591200]
//    240 near summertime end 2020-10-25 06:00:00[2020-10-25 02:00:00 / 1603587600]
//    300 near summertime end 2020-10-25 06:00:00[2020-10-25 02:00:00 / 1603587600]
//    360 near summertime end 2020-10-25 06:00:00[2020-10-25 01:00:00 / 1603580400]    }

?>