I wouldn't use a hidden form element. That just opens the possibility to hack it. Just hard code the current date as the compare field.
The code should be:
$WAFV_Errors .= WAValidateDT((date("m/d/Y")) . "",true,".*","","".$rsPresentkort->getColumnVal("PresentkortValidTo") ."",false,".*","","",true,3);
JQuery validation doesn't work to validate hidden fields (you can't correct the value of a hidden field). It has to know where to place the error based on a hidden field. You should attach that error message to the entered gift card field instead.