qui un pochino di JS fa comodo
codice:
in head:
<script type="text/javascript" language="JavaScript"> 
	function leapto(form) {
	var myindex=form.dest.selectedIndex
	window.location=(form.dest.options[myindex].value);
	}
</script>

in body:
<form action="" method="get" >
	<select  name="dest" >
	<option value="pagina_1.html">periodo...</option>
	<option value="pagina_2.html">periodo...</option>
	</select>
	<input type="button" value="Vai" onClick="leapto(this.form)">
</form>