Fixes DateUtil::validateDate()
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.```
Cyperghost/WCF,Cyperghost/WCF,joshuaruesweg/WCF,SoftCreatR/WCF,Morik/WCF,Morik/WCF,NBZ4live/WCF,cashcat/WCF-1,WoltLab/WCF,SoftCreatR/WCF,SoftCreatR/WCF,PhrozenByte/WCF2,MenesesEvandro/WCF,WoltLab/WCF,NBZ4live/WCF,0xLeon/WCF,PhrozenByte/WCF2,0xLeon/WCF,Morik/WCF,WoltLab/WCF,Cyperghost/WCF,Morik/WCF,Cyperghost/WCF,joshuaruesweg/WCF,Cyperghost/WCF,joshuaruesweg/WCF,MeinAccount/WCF,0xLeon/WCF,SoftCreatR/WCF,MenesesEvandro/WCF,ramiusGitHub/WCF,ramiusGitHub/WCF,WoltLab/WCF,MeinAccount/WCF,cashcat/WCF-1