function setDay(obj) {
  obj = obj.form;
  var years = parseInt(obj.date_year.options[obj.date_year.selectedIndex].value);
  var months = parseInt(obj.date_month.options[obj.date_month.selectedIndex].value);
  var lastday = monthday(years,months);
  var itemnum = obj.date_day.length;
  if (lastday - 1 < obj.date_day.selectedIndex) {
    obj.date_day.selectedIndex = lastday - 1;
  }
  obj.date_day.length = lastday;
  for (cnt = itemnum + 1; cnt <= lastday; cnt++) {
    obj.date_day.options[cnt - 1].text = cnt;
  }
}

function monthday(years, months) {
  var lastday = new Array(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
  if (((years % 4 == 0) && (years % 100 != 0)) || (years % 400 == 0)) {
    lastday[1] = 29;
  }
  return lastday[months - 1];
}