<html>
<head>
<script type="text/javascript">
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
t = setTimeout('startTime()', 500)
}
function checkTime(i) {
if (i < 10) {
i = "0" + i
}
return i
}
</script>
</head>
<body onload="startTime()">
<div id="txt"></div>
</body>
</html>参考：
http://www.w3school.com.cn/tiy/t.asp?f=hdom_timing_clock