"Schmutziger" Schnellschuss für eine mögliche Lösung:
<html>
<form id="formular" onsubmit="setTimeValue()">
<input id="h" type="number" min=0 max=23 step=1 style="width:3em">:<select id="m"><option value=0>00</option><option value=.25>15</option><option value=.5>30</option><option value=.75>45</option></select>
<input id="t" name="t" type="hidden" />
<button>foo</button>
</form>
<script>
function setTimeValue () {
document.getElementById('t').value = document.getElementById('h').value * 3600 + 3600 * document.getElementById('m').value;
}
</script>
</html>