ciao grazie della risposta..

ecco il resto del codice che uso.


codice:
<html>
<head>
<script type="text/javascript">
function totaltime()
{

    var i2, campo1, campo2,  totalhours, totalminutes, finaltotalhours, addhours, realminutes;
	totalhours=0;
	totalminutes=0;
    for (i2=1; i2<=10; i2++){
        campo1 = document.getElementById("hh"+i2).value;
        campo2 = document.getElementById("mm"+i2).value;
		if (campo1==''){campo1=0;}
		if (campo2==''){campo2=0;}
		totalhours=(totalhours + parseInt(campo1));
		totalminutes=parseInt(totalminutes+parseInt(campo2));
        }
    
addhours= parseInt(totalminutes / 60);
realminutes= totalminutes-(addhours*60);
finaltotalhours=totalhours+addhours;
if (realminutes<10){realminutes='0'+realminutes;}
document.getElementById("time").innerHTML = parseInt(finaltotalhours)+':'+realminutes;
}
</script>
</head>
<body>
<form>
<input type=textbox onkeyup="if(this.value.length==1)this.form.mm1.focus(); totaltime();" size=1 id="hh1" name="hh1">:<input type=textbox onkeyup="if(this.value.length==2)this.form.hh2.focus(); totaltime();" size=2 id="mm1" name="mm1">
<input type=textbox onkeyup="if(this.value.length==1)this.form.mm2.focus(); totaltime();" size=1 name=hh2>:<input type=textbox onkeyup="totaltime();" size=2 name=mm2>
</form>

Somma ore: <div id="time"> </div>
</body>
</html>