dunque, ho trovato un cronometro, ma vorrei che mi segnalasse solo i millisecondi (senza ore e minuti) e in piu' che il cronometro si avvi con l'apertura di una immagine. come faccio?
<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">




<input type=button name="start" value="Start/Lap" onclick="sw_start()">
<input type=button name="reset" value="Reset" onclick="Reset()">
</form>