Description:
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When in a timezone that's ahead of UTC, DateTime->diff seems to have issues when the starting date falls on a leap day in UTC.

In the test script, changing the dates from March 1 to March 2 has both var_dump calls showing 10.

Test script:
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<?php

date_default_timezone_set('UTC');

$d = new DateTime('2008-03-01');
$a = new DateTime('2018-03-01');

var_dump($d->diff($a)->y);

date_default_timezone_set('Europe/Amsterdam');

$d = new DateTime('2008-03-01');
$a = new DateTime('2018-03-01');

var_dump($d->diff($a)->y);

Expected result:
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int(10)
int(10)

Actual result:
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int(10)
int(9)