DateTime::diff() miscounts hours in days where a DST transition happens

[Fischer-Bjoern]

Description

The following code:

<?php

$ref = new DateTime('2024-01-01 00:00:00', new DateTimeZone('Europe/Berlin'));
$t1 = new DateTime('2024-03-31 12:00:00', new DateTimeZone('Europe/Berlin'));
$t2 = new DateTime('2024-04-01 12:00:00', new DateTimeZone('Europe/Berlin'));
$t3 = new DateTime('2024-10-27 12:00:00', new DateTimeZone('Europe/Berlin'));
$t4 = new DateTime('2024-10-28 12:00:00', new DateTimeZone('Europe/Berlin'));

// DST start
$diff1 = $ref->diff($t1);
$diff2 = $ref->diff($t2);
// DST end
$diff3 = $ref->diff($t3);
$diff4 = $ref->diff($t4);

echo('DST start' . PHP_EOL);
var_dump($diff1->h, $diff2->h);

echo('DST end' . PHP_EOL);
var_dump($diff3->h, $diff4->h);

Resulted in this output:

DST start
int(11)
int(12)
DST end
int(12)
int(12)

But I expected this output instead:

DST start
int(12)
int(12)
DST end
int(12)
int(12)

https://3v4l.org/VkM09

As far as I understand it DateTime::diff() is supposed to count the time based on the local clock of a time zone.
But on the day of a forward time transition when DST starts (2024-03-31), DateTime::diff() only counts 11 hours when the time on the local clock is actually 12 o'clock.

This is also inconsistent with the day of a backwards time transition when DST ends (2024-10-27), where DateTime::diff() still counts 12 hours when actually 13 hours have elapsed.

This issue started with PHP version 8.1. Prior versions work correctly.

PHP Version

PHP 8.1.32 (cli) (built: Mar 13 2025 18:27:44) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.32, Copyright (c) Zend Technologies
with Zend OPcache v8.1.32, Copyright (c), by Zend Technologies

Operating System

Ubuntu 24.04.1

[carlosbuenosvinos]

It also fails without the time zones defined:

https://3v4l.org/cpPdj

[Fischer-Bjoern]

It also fails without the time zones defined:

Yes, because 3v4l.org uses 'Europe/Amsterdam' as its system time zone, which behaves similar to 'Europe/Berlin'.

The following code:

var_dump(date_default_timezone_get());

Results in this output:

string(16) "Europe/Amsterdam"

https://3v4l.org/ceLtm

But since the time zone used is important for the result, I did not want to rely on some default.

For example with 'UTC' (https://3v4l.org/1osN2) or 'America/Los_Angeles' (https://3v4l.org/K0uNN) the hours are correctly counted as 12, as in those time zones no forward time transition happens on 2024-03-31 at 2 o'clock.

[carlosbuenosvinos]

I'm working on the fix, let's see if I can make it happen :)

[derickr]

@carlosbuenosvinos Make sure you make patches against https://github.com/derickr/timelib — there is also a large set of tests there too. This isn't going to be a trivial one to do cleanly.