js实现倒计时

[html]
<html>
    <head>
        <title>倒计时</title>
        
        <style type= "text/css">
            .daojishi h2
            {
                font-family:Helvetica, Microsoft YaHei, Arial, sans-serif;
                font-size:14px;
                margin-bottom:5px;
                color:#151515;
            }
            .daojishi #timer
            {
                font-family:Helvetica, Microsoft YaHei, Arial, sans-serif;
                font-size:14px;
                color:#151515;
                font-weight:bold;
            }
        </style>
        <script type = "text/javascript" src = "timer.js">
        </script>
    </head>
    <body onload = "timer()">
        <div class = "daojishi">
            <h2>剩余时间为：</h2>
            <div id = "timer">
            </div>
        </div>
    </body>
</html>

timer.js

[javascript]
function timer()
            {
                var ts = (new Date(2012, 11, 11, 9, 0, 0)) - (new Date());//计算剩余的毫秒数
                var dd = parseInt(ts / 1000 / 60 / 60 / 24, 10);//计算剩余的天数
                var hh = parseInt(ts / 1000 / 60 / 60 % 24, 10);//计算剩余的小时数
                var mm = parseInt(ts / 1000 / 60 % 60, 10);//计算剩余的分钟数
                var ss = parseInt(ts / 1000 % 60, 10);//计算剩余的秒数
                dd = checkTime(dd);
                hh = checkTime(hh);
                mm = checkTime(mm);
                ss = checkTime(ss);
                document.getElementById("timer").innerHTML = dd + "天" + hh + "时" + mm + "分" + ss + "秒";
                setInterval("timer()",1000);
            }
            function checkTime(i)
            {
               if (i < 10) {
                   i = "0" + i;
                }
               return i;
            }

补充：web前端 , JavaScript ,