<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<title></title>
<script type ="text/javascript">
var time = 10;
var id=0;
window.onload = function () {
//时钟每次都会返回一个ID
id = setInterval(daojishi, 1000);
setTimeout(daojishi,1000);//当window.onload后，再隔1000ms调用指定的方法
}
function daojishi()
{
time--;
document.getElementById("time").innerHTML=time;
if(time==0)
{
clearInterval(id);//清除定时器
}
}
</script>
</head>
<body>
倒计时 <span id="time"></span>秒
</body>
</html>
原文：http://www.cnblogs.com/sumg/p/3787758.html