function makeValidDate() {
  year = document.theForm.selYear.options[ document.theForm.selYear.selectedIndex ].value;
  month = document.theForm.selMonth.options[ document.theForm.selMonth.selectedIndex ].value;
  day = document.theForm.selDay.options[ document.theForm.selDay.selectedIndex ].value;
  maxDay = 31;
  if ( month == 4 || month == 6 || month == 9 || month == 11 ) {
    maxDay = 30;
  } else if ( month == 2 ) {
    if ( year%100 != 0 && year%4 == 0 ) {
maxDay = 29;
} else {
maxDay = 28;
}
  }
  document.theForm.selDay.selectedIndex = Math.min(day, maxDay)-1;
}