Altro metodo....

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
</head>
<body>
<script language="JavaScript" type="text/javascript">
<!--
oraI = 14.45;
oraF = 17.00;
//
minI = (oraI - parseInt(oraI))*100;
minF = (oraF - parseInt(oraF))*100;

i = parseInt(oraI) * 60 + minI;
f = parseInt(oraF) * 60 + minF;

diff = f - i;
diff = Math.round(diff,0);
ore  = parseInt(diff / 60);
min  = Math.round((diff / 60 - ore)*60);
alert(ore + " ore - " + min + " minuti");
//-->
</script>

</body>
</html>