DateUtil::checkDate() now validates all ISO dates
DateUtil::checkDate() now validates all ISO dates

Fixes #1677

Cyperghost/WCF,SoftCreatR/WCF,SoftCreatR/WCF,MeinAccount/WCF,SoftCreatR/WCF,Cyperghost/WCF,Morik/WCF,cashcat/WCF-1,ramiusGitHub/WCF,0xLeon/WCF,PhrozenByte/WCF2,WoltLab/WCF,NBZ4live/WCF,Morik/WCF,cashcat/WCF-1,NBZ4live/WCF,WoltLab/WCF,PhrozenByte/WCF2,Cyperghost/WCF,Morik/WCF,MenesesEvandro/WCF,0xLeon/WCF,Cyperghost/WCF,MeinAccount/WCF,joshuaruesweg/WCF,Cyperghost/WCF,joshuaruesweg/WCF,MenesesEvandro/WCF,WoltLab/WCF,SoftCreatR/WCF,0xLeon/WCF,ramiusGitHub/WCF,joshuaruesweg/WCF,Morik/WCF,WoltLab/WCF