String.prototype.trim = function () {
	return this.replace(/(^\s+)|(\s+$)/, '');
}
String.prototype.addTrailingSlash = function () {
	return ('/' != this.charAt(this.length - 1)) ? this.toString() + '/' : this.toString();
}
String.prototype.left = function (n) {
	if (n <= 0) return "";
	else if (n > String(this).length) return this;
	else return String(this).substring(0,n);
}
String.prototype.right = function (n){
    if (n <= 0) return "";
    else if (n > String(this).length) return str;
    else {
       var iLen = String(this).length;
       return String(this).substring(iLen, iLen - n);
    }
}

// Declaring valid date character, minimum year and maximum year
var dtCh = "/";
var minYear = 1900;
var maxYear = 2100;
 
function isInteger(s) {
	var i;
	for (i = 0; i < s.length; i++) {
		// Check that current character is number.
		var c = s.charAt(i);
		if (((c < "0") || (c > "9"))) return false;
	}
	// All characters are numbers.
	return true;
}
 
function stripCharsInBag (s, bag){
	var i;
	var returnString = "";
	// Search through string's characters one by one.
	// If character is not in bag, append to returnString.
	for (i = 0; i < s.length; i++) {
		var c = s.charAt(i);
		if (bag.indexOf(c) == -1) returnString += c;
	}
	return returnString;
}
 
function daysInFebruary (year){
	// February has 29 days in any year evenly divisible by four,
	// EXCEPT for centurial years which are not also divisible by 400.
	return (((year % 4 == 0) && ( (!(year % 100 == 0)) || (year % 400 == 0))) ? 29 : 28 );
}
 
function DaysArray(n) {
	for (var i = 1; i <= n; i++) {
		this[i] = 31
		if (i==4 || i==6 || i==9 || i==11) {this[i] = 30}
		if (i==2) {this[i] = 29}
	}
	return this;
}
 
function isDate(dtStr) {
	var daysInMonth = DaysArray(12);
	var pos1 = dtStr.indexOf(dtCh);
	var pos2 = dtStr.indexOf(dtCh, pos1 + 1);
	var strMonth = dtStr.substring(0, pos1);
	var strDay = dtStr.substring(pos1 + 1, pos2);
	var strYear = dtStr.substring(pos2 + 1);
	strYr = strYear;
	if (strDay.charAt(0) == "0" && strDay.length > 1) strDay = strDay.substring(1);
	if (strMonth.charAt(0)=="0" && strMonth.length > 1) strMonth = strMonth.substring(1);
	for (var i = 1; i <= 3; i++) {
		if (strYr.charAt(0)=="0" && strYr.length > 1) strYr = strYr.substring(1);
	}
	month = parseInt(strMonth);
	day = parseInt(strDay);
	year = parseInt(strYr);
	if (pos1 == -1 || pos2 == -1) {
		alert("The date format should be : mm/dd/yyyy");
		return false;
	}
	if (strMonth.length < 1 || month < 1 || month > 12) {
		alert("Please enter a valid month");
		return false;
	}
	if (strDay.length < 1 || day < 1 || day > 31 || (month == 2 && day > daysInFebruary(year)) || day > daysInMonth[month]) {
		alert("Please enter a valid day");
		return false;
	}
	if (strYear.length != 4 || year == 0 || year < minYear || year > maxYear) {
		alert("Please enter a valid 4 digit year between " + minYear + " and " + maxYear);
		return false;
	}
	if (dtStr.indexOf(dtCh, pos2 + 1) != -1 || isInteger(stripCharsInBag(dtStr, dtCh)) == false) {
		alert("Please enter a valid date");
		return false;
	}
	return true;
}

function validateDate(dateValue) {
	var RegExPattern = /^(?=\d)(?:(?:(?:(?:(?:0?[13578]|1[02])(\/|-|\.)31)\1|(?:(?:0?[1,3-9]|1[0-2])(\/|-|\.)(?:29|30)\2))(?:(?:1[6-9]|[2-9]\d)?\d{2})|(?:0?2(\/|-|\.)29\3(?:(?:(?:1[6-9]|[2-9]\d)?(?:0[48]|[2468][048]|[13579][26])|(?:(?:16|[2468][048]|[3579][26])00))))|(?:(?:0?[1-9])|(?:1[0-2]))(\/|-|\.)(?:0?[1-9]|1\d|2[0-8])\4(?:(?:1[6-9]|[2-9]\d)?\d{2}))($|\ (?=\d)))?(((0?[1-9]|1[012])(:[0-5]\d){0,2}(\ [AP]M))|([01]\d|2[0-3])(:[0-5]\d){1,2})?$/;
	if ((dateValue.match(RegExPattern)) && dateValue.length >= 8 && dateValue.length <= 10) {
		return true;
	} else {
		return false;
	}
}

function validDate(date){
	//split the date using ANY separator
	var re = /(\d+)\D+(\d+)\D+(\d+)/;
	var a = date.match(re);
	if (a) {
		//check the year format
		if (a[3].length==2) a[3]= (a[3] > 69 ? '19' : '20') + a[3];
		if (a[3].length < 4) return false;
 
		//use integers and avoid octal parse errors for numbers starting with '0'
		a[1] = parseInt(a[1], 10);
		a[2] = parseInt(a[2], 10);
		a[3] = parseInt(a[3], 10);
 
		//use the Javascript Date object to validate the date
		var dt = new Date();
		dt.setFullYear(a[3]);
		dt.setMonth(a[1] - 1);
		dt.setDate(a[2]);
		if((a[1]==dt.getMonth() + 1) && a[2]==dt.getDate() && a[3]==dt.getFullYear())
			return (String(a[1]).length > 1 ? a[1] : '0' + a[1]) + '/' + (String(a[2]).length > 1 ? a[2] : '0' + a[2]) + '/' + a[3];
		else return null;
	}
	else return null;
}

<input type="text" name="datefield" value="" onblur="if (!(this.value=validDate(this.value))) alert('Please enter a valid date.');" />