This javascript cut & paste code snippet checks if a date entered is in valid Month/Date/Year format. An excellent use of regular expressions for the initial validation routine as well. Also includes illogical check for invalid dates such as February 29 if it is not a leap year.

Insert this code into your HEAD of HTML

<!-- // == This Script Free To Use Providing This Notice Remains == // // == This Script Has Been Found In The http://www.DesignerWiz.com Javascript Public Archive Library == // // == NOTICE: Though This Material May Have Been In A Public Depository, Certain Author Copyright Restrictions May Apply == // --><script language="JavaScript" type="text/javascript"> <!-- Original: Sandeep V. Tamhankar (stamhankar@hotmail.com) --> <!-- Begin 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[1]; // parse date into variables day = matchArray[3]; year = matchArray[4]; 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 } // End --> </script>

Insert this code into your BODY section

<center> <form onSubmit="return isValidDate(this.date.value);"> Example: Your Birth Date: <input type=text name=date size=10 maxlength=10> (in MM/DD/YYYY format) <input type="submit" value="Submit Date"> </form> </center>

Simply Cut and Paste Script code
Be sure to properly place script position in HEAD & BODY sections of html 

This script has been in the DesignerWiz.com Public Library.