以下代码亲自测试
<span style="font-size:14px;"><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>JS实现倒计时(时、分，秒)</title>
<SCRIPT LANGUAGE="JavaScript">
var maxtime = 10 //一个小时，按秒计算，自己调整!
function CountDown(){
if(maxtime>=0){
minutes = Math.floor(maxtime/60);
seconds = Math.floor(maxtime%60);
msg = "距离结束还有"+minutes+"分"+seconds+"秒";
document.all["timer"].innerHTML=msg;
if(maxtime == 5*60) alert('注意，还有5分钟!');
--maxtime;
}
else{
clearInterval(timer);
alert("时间到，结束!");
}
}
timer = setInterval("CountDown()",1000);
</SCRIPT>
<div id="timer" style="color:red" mce_style="color:red"></div>
</body>
</html>
</span>原文：http://blog.csdn.net/u013047660/article/details/45097491