Guarda l'esempio:
codice:
<HTML>
<HEAD>
<TITLE> &copy; br1 - 2007 </TITLE>
<script>
var old_i = 0;
var old_f = 0;
function controlla(obj) {
	f = obj.form;
	n = obj.name;
	new_i = f.inizio.selectedIndex;
	pippo = f.inizio.options[new_i].value.split("/");
	data_i = new Date(pippo[2],pippo[1]-1,pippo[0]);
	new_f = f.fine.selectedIndex;
	pippo = f.fine.options[new_f].value.split("/");
	data_f = new Date(pippo[2],pippo[1]-1,pippo[0]);
	if(data_f<data_i) {
		alert("Data inizio > data fine");
		f.inizio.selectedIndex = old_i;
		f.fine.selectedIndex = old_f;
	} else {
		old_i = new_i;
		old_f = new_f;
	}
}
</script>
</HEAD>
<BODY>
<form>
<select name="inizio" onchange="controlla(this)">
<option value="19/07/2007">19/07/2007</option>
<option value="19/08/2007">19/08/2007</option>
<option value="19/09/2007">19/09/2007</option>
</select>
<select name="fine" onchange="controlla(this)">
<option value="29/07/2007">29/07/2007</option>
<option value="29/08/2007">29/08/2007</option>
<option value="29/09/2007">29/09/2007</option>
</select>
</form>
</BODY>
</HTML>
ciao