代码如下：
<body>
<div><span id="hh">00</span>:<span id="mm">00</span>:<span id="ss">00</span></div>
<script>
window.onload=function(){
var h=0;//时
var m=0;//分
var s=10;//秒
var count=h*3600+m*60+s;
var timer;
timer=setInterval(function(){
count--;
if(count>=0) {
$("#hh").text(showNum(Math.floor(count / 3600)));
$("#mm").text(showNum(Math.floor(count / 60)));
$("#ss").text(showNum(count % 60))
}else{
//do something
alert("时间到了");
clearInterval(timer);//取消计时，不加这句会一直循环
}
},1000);
}
function showNum(num){
if(num<10)
return '0'+num;
return num;
}
</script>
</body>
