codice:
<form name="tuoForm">

Inizio: <input type="text" name="ora1"> Fine: <input type="text" name="ora2">
<input type="checkbox" name="mensa"> Mensa</p></form>
<script type="text/javascript">
document.tuoForm.ora1.onchange = document.tuoForm.mensa.onchange = function () {
  document.tuoForm.ora2.value = (((parseInt(document.tuoForm.ora1.value) + 6) % 24).toString() + document.tuoForm.ora1.value.replace(/^[^\:]*/, "")).replace(/(?:\:\d+)?$/, document.tuoForm.mensa.checked ? ":30" : ":00");
};
</script>