function isInteger(sInteger) {

    var isInt = true;
    inputStr = sInteger.toString(); // in case not a string already
    for (var i = 0; i < inputStr.length; i++) {
        var oneChar = inputStr.charAt(i);
        if (oneChar < "0" || oneChar > "9") {
            isInt = false;
            i = inputStr.length; // break out of loop when bad char found
        }
    } return isInt;
}

/*
 * Validates date strings.
 *
 * Parameter: field    - field containing the date string
 * Internal Calls: isInteger() and isNotBlank()
 * Returns:   true (valid date) or false (not valid) boolean
 */

// Declaring valid date character, minimum year and maximum year
var dtCh= "/";
var minYear=2000;
var maxYear=2020;


function isDate(field) {
    if(field.value=="")
    	return true;
    
    var valid = true;
    var sDate = field.value;
    var Slash1Pos = sDate.indexOf("/",0);
    var Slash2Pos = sDate.indexOf("/",Slash1Pos + 1);
    var dd = sDate.substring(0,Slash1Pos);
    var mm = sDate.substring(Slash1Pos + 1,Slash2Pos);
    var yyyy = sDate.substring(Slash2Pos + 1,sDate.length);

    // Validate date
    if (isInteger(mm) == false || isInteger(dd) == false || isInteger(yyyy) == false)
        valid = false;

    if (yyyy.length != 4)
        valid = false;
    else if (mm < 1 || mm > 12)
        valid = false;
    else if (dd < 1 || dd > 31)
        valid = false;
    else if (mm == 2) {
        if (dd > 29)
            valid = false ;
        else if (dd == 29) {
            if (yyyy % 100 == 0 && yyyy % 400 != 0)
                valid = false;
            else if (yyyy % 4 != 0)
                valid = false;
        }
    } else if (mm == 4 || mm == 6 || mm == 9 || mm == 11) {
        if (dd > 30)
            valid = false;
    }

    if (valid == false) {
        alert("Invalid date entered.\nPlease enter the date in dd/mm/yyyy format.");
        field.focus();
        field.select();
    }

    return valid;
}