molto primitivo...

<script>
ora1="18:55";
ora2="20:00";
ora1=ora1.split(":");
ora2=ora2.split(":");
h=ora2[0]-ora1[0];
m=ora2[1]-ora1[1];
diff=parseInt(h*60+m);
alert("Diff: "+diff+" min")
</script>

:bubu: