Description:
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DateTimeImmutable and DateTime with a time like '2021-03-28 03:00:00' in 'Europe/Amsterdam' (i.e. the moment of the DST-transition) will give the wrong time when substracting less than an hour. 

The same goes for all times that will give a result within an hour from 03:00+02:00.

I.e. when substracting 30 minutes, you get the same time as when _adding_ (+60-30 = ) 30 minutes.

And the same when using DateTimeImmutable. The before-output should be 01:30+01:00.

https://3v4l.org/ZZjvq (DateTimeImmutable)
https://3v4l.org/v3u2v (DateTime)

Btw substracting 60 minutes gives the exact same time again (03:00:00+02:00), while substracting 61 minutes correctly gives "2021-03-28T00:59:00+01:00".

Test script:
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$date = new DateTime('2021-03-28 03:00:00', new DateTimeZone('Europe/Amsterdam'));
$_30mbefore = (clone $date)->sub(new DateInterval('PT30M'));
$_30mafter = (clone $date)->add(new DateInterval('PT30M'));

var_dump($date->format(DATE_ATOM));
var_dump($_30mbefore->format(DATE_ATOM));
var_dump($_30mafter->format(DATE_ATOM));



Expected result:
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string(25) "2021-03-28T03:00:00+02:00"
string(25) "2021-03-28T01:30:00+01:00" (correct)
string(25) "2021-03-28T03:30:00+02:00"

Actual result:
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string(25) "2021-03-28T03:00:00+02:00"
string(25) "2021-03-28T03:30:00+02:00" (incorrect)
string(25) "2021-03-28T03:30:00+02:00"