Essayez-le vous-même - JavaScript Reference win_settimeout

<!DOCTYPE html>
<html>
<body onload="startTime()">

<div id="txt"></div>

<script>
function startTime() {
   var today = new Date();
   var h = today.getHours();
   var m = today.getMinutes();
   var s = today.getSeconds();
   // add a zero in front of numbers<10
   m = checkTime(m);
   s = checkTime(s);
   document.getElementById("txt").innerHTML = h + ":" + m + ":" + s;
   var t = setTimeout(function(){ startTime() }, 500);
}

function checkTime(i) {
   if (i < 10) {
   i = "0" + i;
   }
   return i;
}
</script>

</body>
</html>