第一种：有开始时间和结束时间的
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>倒计时js代码</title>
</head>
<body>
<DIV id="CountMsg" class="HotDate">
<span id="t_d">00天</span>
<span id="t_h">00时</span>
<span id="t_m">00分</span>
<span id="t_s">00秒</span>
</DIV>
<script type="text/javascript">
function getRTime(){
var EndTime= new Date(‘2013/05/1 10:00:00‘); //截止时间
var NowTime = new Date();
var t =EndTime.getTime() - NowTime.getTime();
var d=Math.floor(t/1000/60/60/24);
var h=Math.floor(t/1000/60/60%24);
var m=Math.floor(t/1000/60%60);
var s=Math.floor(t/1000%60);
document.getElementById("t_d").innerHTML = d + "天";
document.getElementById("t_h").innerHTML = h + "时";
document.getElementById("t_m").innerHTML = m + "分";
document.getElementById("t_s").innerHTML = s + "秒";
}
setInterval(getRTime,1000);
</script>
</body>
</html>
第二种是没有开始时间和结束时间的
<!doctype html>
<html>
<head>
<title>倒计时js代码</title>
</head>
<body>
<script type="text/javascript">
var maxtime = 60 * 60 //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);
minutes = minutes >= 10 ? minutes : ‘0‘ + minutes;
seconds = seconds >= 10 ? seconds : ‘0‘ + seconds;
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>
</body>
<div id="timer" style="color:red"></div>
</html>
请自测试，没问题啦！
原文：http://www.cnblogs.com/LoveQin/p/5060806.html