function isValidDate(dateStr) {
// Checks for the following valid date formats:
// MM/DD/YY   MM/DD/YYYY   MM-DD-YY   MM-DD-YYYY
// Also separates date into month, day, and year variables

var datePat = /^(\d{1,2})(\/)(\d{1,2})\2(\d{2}|\d{4})$/;

// To require a 4 digit year entry, use this line instead:
// var datePat = /^(\d{1,2})(\/|-)(\d{1,2})\2(\d{4})$/;

var matchArray = dateStr.match(datePat); // is the format ok?
	if (matchArray == null) {
		alert("Date is not in a valid format.")
		return false;
	}
	
	month = matchArray[3]; // parse date into variables
	day = matchArray[1];
	year = matchArray[4];

	if (day < 1 || day > 31) {
		alert("Day must be between 1 and 31.");
	return false;
	}

	if (month < 1 || month > 12) { // check month range
		alert("Month must be between 1 and 12.");
		return false;
	}
	
	if (year < 1921 || year > 2020){
		 alert("Year must be between 1921 and 2020.");
		 return false;
	}
	
	if ((month==4 || month==6 || month==9 || month==11) && day==31) {
		alert("Month "+month+" doesn't have 31 days!")
		return false
	}

	if (month == 2) { // check for february 29th
		var isleap = (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
		if (day>29 || (day==29 && !isleap)) {
			alert("February " + year + " doesn't have " + day + " days!");
			return false;
	   }
	}
	
	return true;  // date is valid
}