... OK, try the following javascript and let me know how you get on?
function ValidateMonthandYear() {
debugger;
var bReturn=true;
var monthformat = { 'Jan': 01, 'Feb': 02, 'Mar': 03, 'Apr': 04, 'May': 05, 'Jun': 06, 'Jul': 07, 'Aug': 08, 'Sep': 09, 'Oct': 10, 'Nov': 11, 'Dec': 12 };
document.getElementById('<%=lblDateofPurchaseValidation.ClientID%>').innerHTML = '';
document.getElementById('<%=lblMonthandYearofManufacturingvalidation.ClientID%>').innerHTML = '';
var Manufyear = document.getElementById('<%=ddlManufatureyear.ClientID%>').value ;
var Manufmonth = document.getElementById('<%=ddlManufaturemonth.ClientID%>').value;
var ManufDay = document.getElementById('<%=ddlManufatureDay.ClientID%>').value;
if (Manufyear != "0" && Manufmonth != "0" && ManufDay != "0") {
var date = Manufyear - +Manufmonth - +ManufDay;
}
else {
bReturn = false;
}
return bReturn;
}