Вариант с прямым или обратным отсчётом ...
<html>
<head>
<meta charset="utf-8">
<style type="text/css">
body { background-color:#ffcc33; margin:0px; }
div { color:#0033cc; font:bold 12pt Tahoma; margin:15px; }
</style>
<script type="text/javascript">
var t = new Date,f = 1,s,n;
t.setHours(0, 0, 0, 0);
function Timer() {
t = new Date(t.getTime() + f*((new Date).getTime() - s.getTime()));
document.getElementsByTagName("div")[0].innerHTML = t.toLocaleTimeString();
s = new Date;
n = setTimeout(arguments.callee, 500)
}
function New_start() {
t.setHours(0, 0, 0, 0);
s = new Date;
Timer()
}
function Pause() {
s ? (window.clearTimeout(n), s = !1) : (s = new Date, Timer())
};
function BackTimer()
{
f = -f
}
</script>
</head>
<body>
<input type="button" onclick="New_start()" value="Start/New">
<input type="button" onclick="Pause();" value="Pause/Go">
<input type="button" onclick="BackTimer();" value="BackTimer">
<div>Div HTML</div>
</body>
</html>