function isValidDate(dateStr) {
//var datePat = /^(\d{1,2})(\/|-)(\d{1,2})\2(\d{2}|\d{4})$/;
var datePat = '([0-9]{4})-([0-9]{2})-([0-9]{2})';

// 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[2]; // parse date into variables
day = matchArray[3];
year = matchArray[1];
if (month < 1 || month > 12) { // check month range
//alert("Month must be between 1 and 12.");
return false;
}
if (day < 1 || day > 31) {
//alert("Day must be between 1 and 31.");
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
}