function checkdate(form)
{
var stemp=form.sMonth.options[form.sMonth.selectedIndex].value;
select case stemp
case 1,3,5:MaxDay=31;
case 2:MaxDay=29;
default:MaxDay=30;
form.sDay.length=maxday;
}
in jsp:
<select name="sMonth" onChange="checkDate(this)">
<select name="sDay">
<%for i=1 to maxday%>
<option value=i><%=i></option>
<%next%>