Méthode : Date.getMilliseconds()

<script type="text/javascript">
function getTimestamp() {
 var dt=new Date();
 var h=dt.getHours();
 h = h>=10 ? h : "0"+h;
 var m=dt.getMinutes();
 m = m>=10 ? m : "0"+m;
 var s=dt.getSeconds();
 s = s>=10 ? s : "0"+s;
 var ms=dt.getMilliseconds();
 ms = "00"+ms;
 ms = ms.substring(ms.length-3,ms.length);
 return h+":"+m+":"+s+"."+ms;
}

document.write(getTimestamp());
</script>