Date validation using Javascript in asp.net

function validate() {
var oInput1 = document.getElementById("datepicker_Control");

if (isValidDate(oInput1))
{
//proceed code
}
else
{
// alert message
}

}



function isValidDate(field) {
var checkstr = "0123456789";
var DateField = field;
var Datevalue = "";
var DateTemp = "";
var seperator = ",";
var day;
var month;
var year;
var leap = 0;
var err = 0;
var i;
err = 0;
DateValue = DateField;
/* Delete all chars except 0..9 */
for (i = 0; i < DateValue.length; i++) { if (checkstr.indexOf(DateValue.substr(i, 1)) >= 0) {
DateTemp = DateTemp + DateValue.substr(i, 1);
}
}
DateValue = DateTemp;
/* Always change date to 8 digits - string*/
/* if year is entered as 2-digit / always assume 20xx */
if (DateValue.length == 6) {
DateValue = DateValue.substr(0, 4) + '20' + DateValue.substr(4, 2);
}
if (DateValue.length != 8) {
err = 19;
}
/* year is wrong if year = 0000 */
year = DateValue.substr(4, 4);
if (year == 0) {
err = 20;
}
/* Validation of month*/
month = DateValue.substr(2, 2);
if ((month < 1) || (month > 12)) {
err = 21;
}
/* Validation of day*/
day = DateValue.substr(0, 2);
if (day < 1) { err = 22; } /* Validation leap-year / february / day */ if ((year % 4 == 0) || (year % 100 == 0) || (year % 400 == 0)) { leap = 1; } if ((month == 2) && (leap == 1) && (day > 29)) {
err = 23;
}
if ((month == 2) && (leap != 1) && (day > 28)) {
err = 24;
}
/* Validation of other months */
if ((day > 31) && ((month == "01") || (month == "03") || (month == "05") || (month == "07") || (month == "08") || (month == "10") || (month == "12"))) {
err = 25;
}
if ((day > 30) && ((month == "04") || (month == "06") || (month == "09") || (month == "11"))) {
err = 26;
}
/* if 00 ist entered, no error, deleting the entry */
if ((day == 0) && (month == 0) && (year == 00)) {
err = 0; day = ""; month = ""; year = ""; seperator = "";
}
/* if no error, write the completed date to Input-Field (e.g. 13.12.2001) */
if (err == 0) {
DateField.value = day + seperator + month + seperator + year;
}
/* Error-message if err != 0 */
else {
return false;
}
return true;
}