In this example the date field is named:
BirthDate
The onBlur javascript event needs to say:
checkdateBD()


function checkdateBD(){
//	window.onerror=null // for all other strange errors
	var err=0
	var psj=0;
	var fff = document.forms[0];
	a=fff.BirthDate.value
		
if (a.length != 10) err=1
	b = a.substring(0, 2)// month
	c = a.substring(2, 3)// '/'
	d = a.substring(3, 5)// day
	e = a.substring(5, 6)// '/'
	f = a.substring(6, 10)// year

	//basic error checking
	if (b<1 || b>12) err = 1
	if (c != '/') err = 1
	if (d<1 || d>31) err = 1
	if (e != '/') err = 1
	if (f<100 || f>9999) err = 1
	
	//advanced error checking

	// months with 30 days
	if (b==4 || b==6 || b==9 || b==11){
		if (d==31) err=1
	}

	// february, leap year
	if (b==2){
		// feb
		var g=parseInt(f/4)
		if (isNaN(g)) {
			err=1
		}

		if (d>29) err=1
		if (d==29 && ((f/4)!=parseInt(f/4))) err=1
	}

	if (err==1){
		alert('The date is an invalid format - please re-enter in your date.');
		fff.BirthDate.value = ""
		fff.BirthDate.focus()
	}

}