倒计时

<span id="clock">00:00:11:00</span>
<input type=button value="start countdown!" onclick="onTimer()">
<input type=button value="stop countdown!" onclick="window.clearTimeout(timer);">
<script language="Javascript">

var elapse = 100;
var start = document.all("clock").innerText;
var finish = "00:00:00:00";
var timer = null;
function onTimer(i)
{

if (start == finish)
{
  window.clearTimeout(timer);
  alert("time is reach!");
  return;
}

var hms = new String(start).split(":");
var ms = new Number(hms[3]);
var s = new Number(hms[2]);
var m = new Number(hms[1]);
var h = new Number(hms[0]);
 
ms -= 10;
if (ms < 0)
{
  ms = 90;
  s -= 1;
  if (s < 0)
  {
    s = 59;
    m -= 1;
  }
 
  if (m < 0)
  {
    m = 59;
    h -= 1;
  }
}

var ms = ms < 10 ? ("0" + ms) : ms;
var ss = s < 10 ? ("0" + s) : s;
var sm = m < 10 ? ("0" + m) : m;
var sh = h < 10 ? ("0" + h) : h;

start = sh + ":" + sm + ":" + ss + ":" + ms;
document.all("clock").innerText = start;

timer = window.setTimeout("onTimer()",elapse);
}

</script>