how to allow Asp TextBox only Date Format using JavaScript?

<script type="text/javascript">

var dtCh = "/";

var minYear = 1900;

var maxYear = 9999;

function  strCharsInBag(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) {

    dtStr = dtStr.substring(0, 10);

    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("  Date format should be : MM/DD/YYYY");

        return false;

    }

    if (strMonth.length < 1 || month < 1 || month > 12) {

        alert("  enter a valid month");

        return false;

    }

    if (strDay.length < 1 || day < 1 || day > 31 || (month == 2 && day > daysInFebruary(year)) || day > daysInMonth[month]) {

        alert("  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 || onlyNumbers(strCharsInBag(dtStr, dtCh)) == false) {

        alert("Please enter a valid date");

        return false;

    }

} 

function DateValidations(evt) {

         var e = event || evt; // for trans-browser compatibility

         var charCode = e.which || e.keyCode;

         if (charCode > 31 && (charCode < 47 || charCode > 57))

           {

             return false;

             }

          else{

         return true;

         }

     }

    function validateDate(input)   

{

       var dt = input.value;

    if (isDate(dt) == false) {

    input.value=""

       input.focus();

        return false;

    }

     }

function onlyNumbers(evt) {

    var e = event || evt; // for trans-browser compatibility

    var charCode = e.which || e.keyCode;

    if (charCode > 31 && (charCode < 48 || charCode > 57))

        return false;

    return true;

}

</script>

In Code Behind:

onchage = "validateDate(this)" , onkeypress="DateValidations()"