知识点：
1、setInterval() 方法可按照指定的周期（以毫秒计）来调用函数或计算表达式。
2、clearInterval() 方法可取消由 setInterval() 设置的 timeout。
clearInterval() 方法的参数必须是由 setInterval() 返回的 ID 值。
代码
<!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=utf-8" />
<title>倒计时</title>
</head>
<script>
var timer = function(id)
{
this.objTimer = document.getElementById(id);
}
timer.prototype =
{
init : function ()
{
var _self = this;
if(!this.objTimer)
return;
this.times = parseInt(this.objTimer.innerHTML);
if(this.times <= 0)
{
this.times = 0;
}
this.setTimer();
this.interval = setInterval(function(){_self.setTimer.call(_self)},1000);
},
setTimer : function ()
{
if(this.times <=0)
{
clearInterval(this.intervar);
return;
}
this.times--;
var timeH = parseInt(this.times / 3600);
var timeM = parseInt(this.times % 3600 / 60);
var timeS = parseInt(this.times % 3600 % 60 );
this.objTimer.innerHTML = timeH +":"+ timeM + ":" +timeS;
}
}
</script>
<body>
<div id="times_sec">1000</div>
<div id="times_sec2">500</div>
<script>
var aa = new timer ("times_sec");
var cc = aa.init();
var bb = new timer ("times_sec2");
var dd = bb.init();
</script>
</body>
</html>