Form onLoad:
var f = document_FrmAlias;

Validation Code:
(e.g. in onChange/onBlur, or a button onClick)
var curval = f.DateFld.value;
var matchre = /^\d{1,2}\/\d{1,2}\/\d{4}$/;
if (curval.match(matchre)) {
	// date is okay, proceed with other checks
	....

} else {
	alert('This date is not in the proper format: mm/dd/yyyy. Please try again.');
	return false;
}