Check if the start date is ealier than the end date 

Verify that the start date is earlier than the end date on the Edit page . Use the following code in the Edit page: Before record updated event.

if (toPHPTime($values["start_date"]) > toPHPTime($values["end_date"]))
{
  $message = "Start date can not be later than End date.";
  return false;
}
else
{
  return true;
}

See also:

•About Date Control API

•"Edit as" settings: Date

•AJAX helper object: setMessage()

•Store the date and time when a record is modified