Description:
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In Netherland's timezone (GMT+1/+2), if I add 23 hours to 29 October 2016 10 o'clock, I get 30 October 8 o'clock because we moved the clock 1 hour backwards.

But when I add 23 hours to 26 March 2016 10 o'clock, why don't I get 10 o'clock as we moved the clock one hour forward? Instead, I obtain 9 o'clock (as no DST change occured).

See here:
https://3v4l.org/uXf8d

Test script:
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echo date_default_timezone_get(); // prints Europe/Amsterdam
echo "\n\n";

$dateTime = new DateTime('2016-10-29 10:00:00'); // back one hour
echo $dateTime->format(DateTime::RFC3339);
echo "\n";
$dateTime->add(new DateInterval('PT23H'));
echo $dateTime->format(DateTime::RFC3339); // prints 2016-10-30T08:00:00+01:00
echo "\n\n";

$dateTime = new DateTime('2016-03-26 10:00:00'); // forward one hour
echo $dateTime->format(DateTime::RFC3339);
echo "\n";
$dateTime->add(new DateInterval('PT23H'));
echo $dateTime->format(DateTime::RFC3339); // prints 2016-03-27T09:00:00+02:00 (expected 10 o'clock)
echo "\n\n";

$dateTime = new DateTime('2016-05-26 10:00:00'); // no DST change
echo $dateTime->format(DateTime::RFC3339);
echo "\n";
$dateTime->add(new DateInterval('PT23H'));
echo $dateTime->format(DateTime::RFC3339); // prints 2016-05-27T09:00:00+02:00