PHP Date after time change in Europe - php

Question

This morning I ran unit tests and they failed.
Tests took offers from last 48 hours by calculating time with:
date('U', time() - 48 * 3600)
When I used:
(new DateTime('-48 hours'))->format('U')
it shown one hour difference.
May the reason be that on sunday in Poland time was moved one hour backwards

Answer 1

I presume your server runs with Europe/Warsaw as default time zone. PHP date/time calculations are often incorrect if they cross DST boundaries, as it's the case here. I suggest you do all maths in UTC and convert from/to local time as needed.
Please compare:
$warsaw = new DateTimeZone('Europe/Warsaw');
$utc = new DateTimeZone('UTC');
$start = new DateTime('2018-10-28 04:30:00', $warsaw);
$start->modify('-4 hours');
var_dump($start);
$start = new DateTime('2018-10-28 04:30:00', $warsaw);
$start->setTimezone($utc);
$start->modify('-4 hours');
$start->setTimezone($warsaw);
var_dump($start);
object(DateTime)#3 (3) {
["date"]=>
string(26) "2018-10-28 00:30:00.000000"
["timezone_type"]=>
int(3)
["timezone"]=>
string(13) "Europe/Warsaw"
}
object(DateTime)#4 (3) {
["date"]=>
string(26) "2018-10-28 01:30:00.000000"
["timezone_type"]=>
int(3)
["timezone"]=>
string(13) "Europe/Warsaw"
}
The Unix timestamp version of your code should be unaffected because a Unix timestamp is a fixed moment in time (thus doesn't switch with time zones).