js 倒计时 已过去时间

页面中的代码：

<strong id="timer" datatime="2012-12-09 10:20:30"></strong>

<script>
    //获取申请时间
    var time_now_server;
    //开始时间
    var time_now_server=$("#timer").attr("datatime");
    time_now_server = time_now_server*1000;
    setTimeout("show_time()",1000);
</script>

JS的代码

//倒计时js 已过去时间
function show_time(){
    var timer = document.getElementById("timer");
    if(!timer){
        return ;
    }
    var time_now,time_distance,str_time;
    var int_day,int_hour,int_minute,int_second;
    var time_now=new Date();
    time_now=time_now.getTime();
    time_distance=time_now-time_now_server;
    if(time_distance>0)
    {
        int_day=Math.floor(time_distance/86400000)
        time_distance-=int_day*86400000;
        int_hour=Math.floor(time_distance/3600000)
        time_distance-=int_hour*3600000;
        int_minute=Math.floor(time_distance/60000)
        time_distance-=int_minute*60000;
        int_second=Math.floor(time_distance/1000)
        //        if(int_hour<10)
        //            int_hour="0"+int_hour;
        //        if(int_minute<10)
        //            int_minute="0"+int_minute;
        if(int_second<10)
            int_second="0"+int_second;
        str_time=int_day+"天"+int_hour+"小时"+int_minute+"分钟"+int_second+"秒";
        timer.innerHTML=str_time;
        setTimeout("show_time()",1000);
    }
    else
    {
        timer.innerHTML =timer.innerHTML;
        clearTimeout(timer)
    }
}