[melky]

<select name="timedo">
        <option value="08">08:00</option>
        <option value="09">09:00</option>
</select>

<iframe src="mydoc.php?id=[B][timedo][/B]" id="frame" width="400" height="400"></iframe> 

<script>
document.querySelector("select[name='timedo']").onchange = function(){document.getElementById("frame").src = "mydoc.php?id="+this.value }
</script>