function rewrite_days()
{
 var days = document.getElementById("day");
 var month = document.getElementById("month");
 var year = document.getElementById("year");
 var days_in_month = new Array(31,28,31,30,31,30,31,31,30,31,30,31);
 if (month.value != 0)
 {
  if ((year.value % 4 == 0) && (month.value == 2))
  {
   days.length = 30;
   days.item(29).value = 29;
   days.item(29).text = 29;
  }
  else
  {
   days.length = days_in_month[month.value - 1] + 1;
   for (var i = 29; i < days.length; i++)
   {
    days.item(i).value = i;
    days.item(i).text = i;
   }
  }
 }
}