根据毫秒计算倒计时

<script type="text/javascript">
    var $countdown = $('#countdown');
    var setTimeLeft;
    window.onload=getLeftTime();
    function getLeftTime(){ 
        var $lefttime = parseInt($countdown.attr('name'));
        if ($lefttime<0) {
            $countdown.text("剩余时间： 00:00:00");
            clearTimeout(setTimeLeft);
        }else{ 
            $lefttime-=1000;
            var leftsecond = parseInt($lefttime/1000);
            var nD = Math.floor(leftsecond/(60*60*24)); //天
            var hour = Math.floor(leftsecond/3600);
            var nH = Math.floor((leftsecond-nD*24*60*60)/3600);  //小时
            var nM = Math.floor((leftsecond-nD*24*60*60-nH*3600)/60); //分钟
            var nS = Math.floor(leftsecond-nD*24*60*60-nH*3600-nM*60); //秒
             hour = (hour<10?'0'+nH:hour);
             nM = (nM<10?'0'+nM:nM);
             nS = (nS<10?'0'+nS:nS);
            $countdown.text("剩余时间: "+hour+":"+nM+":"+nS+"");
            $countdown.attr('name',$lefttime);
            setTimeLeft = setTimeout("getLeftTime()", 1000);
        }
    }

</script>

如果倒计时里面不含有天数，可以只打印hour，如果需要天数的时候，只需要打印出来nD和nH就行了