Javascript Date Validation

<script language = "Javascript">
/**
 * DHTML date validation script. Courtesy of SmartWebby.com (http://www.smartwebby.com/dhtml/datevalidation.asp)
 */
// 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 ValidateForm(){
    var dt=document.frmSample.txtDate
    if (isDate(dt.value)==false){
        dt.focus()
        return false
    }
    return true
 }

</script>

<form name="frmSample" method="post" action="" onSubmit="return ValidateForm()">
                <p>Enter a Date <font color="#CC0000"><b>(mm/dd/yyyy)</b></font>
                  :
                  <input type="text" name="txtDate" maxlength="10" size="15">
                </p>
                <p>
                  <input type="submit" name="Submit" value="Submit">
                </p>
              </form>

function valid_date() {
    var GoDate = document.getElementById("TextBox1");
    var date1 = new Date(GoDate.value);
    var date3 = new Date();
    if (GoDate.value == "") {
        alert("Select The Date");
        return false;
    }
    else if (date1 >= date3) {
    alert("Selected Date should be less than today's date.");
    GoDate.value = "";
    return false;
    }
    else {return true;}
}