[JS] Un semplice cronometro

[threepwood]

[JS] Un semplice cronometro

Sto cercando un semplicissimo cronometro in JS.
Basta che gestisca il conteggio di Ore-minuti-secondi, partendo da 00:00:00.

Non ne ho trovati altri, se non questo che però non riesco a correggere (c'è un problema sull'ora)

Avete qualche script da consigliarmi o riuscite a sistemare questo qua sotto?

<script language="JavaScript">
 
var timercount = 0;
var timestart  = null;
 
function showtimer() {
    if(timercount) {
        clearTimeout(timercount);
        clockID = 0;
    }
    if(!timestart){
        timestart = new Date();
    }
    var timeend = new Date();
    var timedifference = timeend.getTime() - timestart.getTime();
    timeend.setTime(timedifference);
 var minutes_passed = timeend.getMinutes();
 if (minutes_passed==0) minutes_passed =59;
    if(minutes_passed <10){
        minutes_passed = "0" + minutes_passed;
    } 
 var hours_passed = timeend.getHours(); // l'ora qui vale subito "1"...
    if(hours_passed <10){
        hours_passed = "0" + hours_passed;
    }
    var seconds_passed = timeend.getSeconds();
    if(seconds_passed <10){
        seconds_passed = "0" + seconds_passed;
    }
    document.timeform.timetextarea.value = hours_passed + ":" + minutes_passed + ":" + seconds_passed;
    timercount = setTimeout("showtimer()", 1000);
}
 
function sw_start(){
    if(!timercount){
    timestart   = new Date();
    document.timeform.timetextarea.value = "00:59:00";
    timercount  = setTimeout("showtimer()", 1000);
    }
    else{
    var timeend = new Date();
        var timedifference = timeend.getTime() - timestart.getTime();
        timeend.setTime(timedifference);
        var hours_passed = timeend.getHours();
        var minutes_passed = timeend.getMinutes();
 
  if(hours_passed <10){
            hours_passed = "0" + hours_passed;
        }
  if(minutes_passed <10){
            minutes_passed = "0" + minutes_passed;
        }
        var seconds_passed = timeend.getSeconds();
        if(seconds_passed <10){
            seconds_passed = "0" + seconds_passed;
        }
 
        var milliseconds_passed = timeend.getMilliseconds();
        if(milliseconds_passed <10){
            milliseconds_passed = "00" + milliseconds_passed;
        }
        else if(milliseconds_passed <100){
            milliseconds_passed = "0" + milliseconds_passed;
        }
    }
}
 
function Reset() {
    timestart = null;
    document.timeform.timetextarea.value = "00:00:00";
}
 
</script>
<form name="timeform">
Time: <input type=text name="timetextarea" value="00:00" size="10" style = "font-size:20px">
<br>
<br>
<input type=button name="start" value="Start/Lap" onclick="sw_start()">
<input type=button name="reset" value="Reset" onclick="Reset()">
</form>