"Same" DateInterval instances lead to different results, depending on the origin (in combination with DST-switches)

[cziegenberg]

Description

The following code:

<?php
function getIntervalSpecificationFromDateInterval(DateInterval $interval): string
{
    $specification = '';

    if ($interval->invert) {
        $specification .= '-';
    }

    $specification .= 'P';
    $specification .= $interval->y ? $interval->y . 'Y' : '';
    $specification .= $interval->m ? $interval->m . 'M' : '';
    $specification .= $interval->d ? $interval->d . 'D' : '';

    if ($interval->h + $interval->i + $interval->s) {
        $specification .= 'T';
        $specification .= $interval->h ? $interval->h . 'H' : '';
        $specification .= $interval->i ? $interval->i . 'M' : '';
        $specification .= $interval->s ? $interval->s . 'S' : '';
    }

    if ($specification === 'P' || $specification === '-P') {
        $specification = 'PT0S';
    }

    return $specification;
}

$timezone   = new DateTimeZone('Europe/Berlin');
$dstSwitchA = new DateTimeImmutable('2025-03-30 01:00:00', $timezone);
$dstSwitchB = new DateTimeImmutable('2025-10-26 01:00:00', $timezone);
$durationA  = new DateInterval('PT3H');
$durationB  = (new DateTimeImmutable('2025-03-28 01:00:00', $timezone))->diff(
    new DateTimeImmutable('2025-03-28 04:00:00', $timezone),
    true
);
$durationC  = (new DateTimeImmutable('2025-03-28 01:00:00', $timezone))->diff(
    new DateTimeImmutable('2025-03-28 04:00:00', $timezone),
);
$durationD  = (new DateTimeImmutable('2025-03-28 01:00:00', $timezone))->diff(
    new DateTimeImmutable('2025-03-28 04:00:00', $timezone),
);
$durationD  = new DateInterval(getIntervalSpecificationFromDateInterval($durationD));

// Test 1st DST-switch (+1 hour)
echo 'TEST 1 (DST-switch +1 hour)' . PHP_EOL;
$datetimeA = $dstSwitchA->setTimezone($timezone)->add($durationA);
echo 'Constructed:     ' . $datetimeA->format('Y-m-d H:i:s') . PHP_EOL; // expected: 2025-03-30 05:00:00

$datetimeB = $dstSwitchA->setTimezone($timezone)->add($durationB);
echo 'Diff (absolute): ' . $datetimeB->format('Y-m-d H:i:s') . PHP_EOL; // expected: 2025-03-30 05:00:00

$datetimeC = $dstSwitchA->setTimezone($timezone)->add($durationC);
echo 'Diff (relative): ' . $datetimeC->format('Y-m-d H:i:s') . PHP_EOL; // expected: 2025-03-30 05:00:00

$datetimeD = $dstSwitchA->setTimezone($timezone)->add($durationD);
echo 'Workaround:      ' . $datetimeD->format('Y-m-d H:i:s') . PHP_EOL; // expected: 2025-03-30 05:00:00

echo PHP_EOL;

echo 'TEST 2 (DST-switch -1 hour)' . PHP_EOL;

// Test 2nd DST-switch (-1 hour)
// Expected value: 2025-10-26 03:00:00
$datetimeA = $dstSwitchB->setTimezone($timezone)->add($durationA);
echo 'Constructed:     ' . $datetimeA->format('Y-m-d H:i:s') . PHP_EOL;

$datetimeB = $dstSwitchB->setTimezone($timezone)->add($durationB);
echo 'Diff (absolute): ' . $datetimeB->format('Y-m-d H:i:s') . PHP_EOL;

$datetimeC = $dstSwitchB->setTimezone($timezone)->add($durationC);
echo 'Diff (relative): ' . $datetimeC->format('Y-m-d H:i:s') . PHP_EOL;

$datetimeD = $dstSwitchB->setTimezone($timezone)->add($durationD);
echo 'Workaround:      ' . $datetimeD->format('Y-m-d H:i:s') . PHP_EOL;

Resulted in this output:

TEST 1 (DST-switch +1 hour)
Constructed:     2025-03-30 05:00:00
Diff (absolute): 2025-03-30 04:00:00
Diff (relative): 2025-03-30 04:00:00
Workaround:      2025-03-30 05:00:00

TEST 2 (DST-switch -1 hour)
Constructed:     2025-10-26 03:00:00
Diff (absolute): 2025-10-26 04:00:00
Diff (relative): 2025-10-26 04:00:00
Workaround:      2025-10-26 03:00:00

But I expected this output instead:

TEST 1 (DST-switch +1 hour)
Constructed:     2025-03-30 05:00:00
Diff (absolute): 2025-03-30 05:00:00
Diff (relative): 2025-03-30 05:00:00
Workaround:      2025-03-30 05:00:00

TEST 2 (DST-switch -1 hour)
Constructed:     2025-10-26 03:00:00
Diff (absolute): 2025-10-26 03:00:00
Diff (relative): 2025-10-26 03:00:00
Workaround:      2025-10-26 03:00:00

PHP Version

PHP 8.3.16 (cli) (built: Jan 14 2025 20:07:09) (NTS Visual C++ 2019 x64)
Copyright (c) The PHP Group
Zend Engine v4.3.16, Copyright (c) Zend Technologies
    with Zend OPcache v8.3.16, Copyright (c), by Zend Technologies
    with Xdebug v3.4.1, Copyright (c) 2002-2025, by Derick Rethans

Tested with PHP 8.1.0 to 8.4.8

Operating System

No response

[heiglandreas]

Let me try to understand what you try to achieve:

You create one diff (A) with 3 hours hardcoded (PT3H) and one diff (B) from one datetime-object (01:00) minus a second datetime object (04:00). The two datetime objects in question are on either side of a DST transition.

So when DST transitions forward (in March in your case) between 01:00 and 04:00 it moves from 02:00 directly to 03:00, skipping one hour. So the amount of time between the two points in time is 2 hours (not 3 as the mathematical 4-1 would suggest)

And when DST transitions backwards (in October in this case) between 01:00 and 04:00 it moves from 03:00, jumps back to 02:00 and then continues as "normal". So the amount of time between the two points in time is 4 hours (not 3 as the mathematical 4-1 would suggest.

Which means that

• when you add 3 hours via the hardcoded diff (A) to 01:00 right before the march transition you will end up at 05:00 (as the hour between 02:00 and 03:00 is "missing")
• when you add 3 hours via the hardcoded diff (A) to 01:00 right before the october transition you end up ad 03:00 (as the hour between 02:00 and 03:00 is counted twice)

But

• when you add the "dynamic" diff (B) to 01:00 right before the march transition (which will then contain 2 hours) it will end up at 04:00 and
• when you add the "dynamic" diff (B) to 01:00 right before the october transition (which will then contain 4 hours) it will also end up at 04:00

So the result of PHP is to me totally expected and logically correct IMO.

[cziegenberg]

Nearly correct. The diff, which you named "dynamic", was created with date times that are both outside of a DST-switch (but use a timezone with DST-switches), so it contains 3 hours (property "h" set to 3, same as with the "hardcoded" version) in all variants. I retested it and get the same result when I create the "dynamic" variant with UTC.

So you have two DateInterval instances from different sources, both containing a difference of 3 hours and looking the same (except one exported DateInterval property was once set 0, once to false, which didn't seem to matter), but they behave differently when adding (and possibly also when subtracting) date intervals to/from a date time instance and the added/subtraced interval overlaps with added/subtracted times caused by a DST-switch (plus one hour and minus one hour in this case).

This resulted in an error in my code, because a DateInterval (from an unknown source, so it's "hardcoded" or "dynamic") is passed as an argument to a function, and when I work with it I sometimes get the expected result, sometimes not. And they cannot be differed.

[heiglandreas]

Ah. Thanks for the clarification.

I overlooked that the intervals were not created based on the DST-switch but on a date a few days earlier.

So yes! They should all result in the same point in time when the hour is set to the same value.

And indeed they should result in the 05:00 during spring and the 03:00 during the Fall DST.