倒计时原理

    <div class="time">
        <span id="t_d">00天</span>
        <span id="t_h">00时</span>
        <span id="t_m">00分</span>
        <span id="t_s">00秒</span>
    </div>
<script>
   function GetRTime(){
       var EndTime= new Date('2015/06/20 00:00:00');
       var NowTime = new Date();
       var t =EndTime.getTime() - NowTime.getTime();
       var d=Math.floor(t/1000/60/60/24);
       var h=Math.floor(t/1000/60/60%24);
       var m=Math.floor(t/1000/60%60);
       var s=Math.floor(t/1000%60);

       document.getElementById("t_d").innerHTML = d + "天";
       document.getElementById("t_h").innerHTML = h + "时";
       document.getElementById("t_m").innerHTML = m + "分";
       document.getElementById("t_s").innerHTML = s + "秒";
   }
   setInterval(GetRTime,0);
</script>