Fixes DateUtil::validateDate()

Fixes WoltLab/WCF#1283 as gmdate returns a date in GMT but strtotime doesn't if input isn't specified as GMT.

See http://www.php.net/manual/en/function.strtotime.php
> Each parameter of this function uses the default time zone unless a time zone is specified in that parameter.```
Merge pull request #1284 from max-m/patch-2

Fixes DateUtil::validateDate()