Javascript Date validation

I wanted to validate the date.. So i searched for a script and found this one . It wz really useful.
http://www.redips.net/javascript/date-validation/

// check date JavaScript function

// if date is valid then function returns true, otherwise returns false

function isDate(txtDate){

var objDate;  // date object initialized from the txtDate string

var mSeconds; // milliseconds from txtDate

// date length should be 10 characters – no more, no less

if (txtDate.length != 10) return false;

// extract day, month and year from the txtDate string

// expected format is mm/dd/yyyy

// subtraction will cast variables to integer implicitly

var day   = txtDate.substring(3,5)  – 0;

var month = txtDate.substring(0,2)  – 1; // because months in JS start with 0

var year  = txtDate.substring(6,10) – 0;

// third and sixth character should be /

if (txtDate.substring(2,3) != ‘/’) return false;

if (txtDate.substring(5,6) != ‘/’) return false;

// test year range

if (year < 999 || year > 3000) return false;

// convert txtDate to the milliseconds

mSeconds = (new Date(year, month, day)).getTime();

// initialize Date() object from calculated milliseconds

objDate = new Date();

objDate.setTime(mSeconds);

// compare input parameter date and created Date() object

// if difference exists then date isn’t valid

if (objDate.getFullYear() != year)  return false;

if (objDate.getMonth()    != month) return false;

if (objDate.getDate()     != day)   return false;

// otherwise return true

return true;

}

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// how to use isDate() JavaScript function

// this function is called on button click event

function checkDate(){

// define date string to test

var txtDate = document.getElementById(‘txtDate’).value;

// check date and print message

if (isDate(txtDate)) alert(‘OK’);

else alert(‘Invalid date format!’);

}