ASP.NET

function GetDayOfMonth(DayCtrlName, MonthCtrlName, YearCtrlName){
   var DayCtrl=document.getElementById(DayCtrlName);
   var MonthCtrl=document.getElementById(MonthCtrlName);
   var YearCtrl=document.getElementById(YearCtrlName);
   var lastDayValue=DayCtrl.value;
   var DaysCount=31;
   if (MonthCtrl.options[MonthCtrl.selectedIndex].value=='2' && 
       (parseInt(YearCtrl.options[YearCtrl.selectedIndex].value)%4)==0){
       DaysCount=29;
   }else if (MonthCtrl.options[MonthCtrl.selectedIndex].value=='2'){
       DaysCount=28;
   }else if (MonthCtrl.options[MonthCtrl.selectedIndex].value=='4' || 
             MonthCtrl.options[MonthCtrl.selectedIndex].value=='6' || 
             MonthCtrl.options[MonthCtrl.selectedIndex].value=='9' || 
             MonthCtrl.options[MonthCtrl.selectedIndex].value=='11'){
       DaysCount=30;
   }
   DayCtrl.options.length=0;
   var oOption;
   for (var i=1;i<=DaysCount;i++){
       oOption = document.createElement('option');
       DayCtrl.options.add(oOption);
       oOption.innerHTML=i;
       oOption.value=i;
       if (i==lastDayValue) oOption.selected=true;
   }
}