<select name="StartDateMonth">
<option value="1">January</option>
<option value="2">February</option>
<option value="3">March</option>
<option value="4">April</option>
<option value="5">May</option>
<option value="6">June</option>
<option value="7">July</option>
<option value="8">August</option>
<option value="9">September</option>
<option value="10">October</option>
<option value="11">November</option>
<option value="12" selected>December</option>
</select>
<select name="StartDateYear">
<option value="1999">1995</option>
<option value="1999">1996</option>
<option value="1999">1997</option>
<option value="1999">1998</option>
<option value="1999">1999</option>
<option value="2000" selected>2000</option>
<option value="2001">2001</option>
</select>

function showdays(year,month){
 var day="day";
 if(month.value==0){
 document.getElementById(day).length=1;
 return;
 }else{
 if(month.value==1||month.value==3||month.value==5||month.value==7||month.value==8||month.value==10||month.value==12){
   document.getElementById(day).length=1;
   createlist(1,32,day);
   return;
 }else if(month.value==2){
 if(year.value%4==0&&year.value0!=0||year.value%400==0){
 document.getElementById(day).length=1;
 createlist(1,30,day);
 return;
 }else{
 document.getElementById(day).length=1;
 createlist(1,29,day);
 return;
 }
 }else{
 document.getElementById(day).length=1;
 createlist(1,31,day);
 return;
 }
 }
}