PHP is expecting a US date, so 05/16/1969, not 16/05/1969.
PHP assumes dates with slashes are in US format and dates with dashes are EU format. The easiest solution is to use the format: 16-05-1969
Another solution is to make that change in the validation code before it is validated like:
$WAFV_Errors .= WAValidateDT(((isset($_POST["DOB"]))?str_replace("/","-",$_POST["DOB"]):"") . "",true,"\b([12]\d|3[0-1]|0[1-9])-(1[0-2]|0[1-9])-\d{4}\b","","",false,".*","","",true,5);