js倒计时（可定义截止日期）

 <div id="times_wrap" class="time_num">
     距离结束时间：
     <div class="time_w">
         <b id="times_d" class="time"> </b>天
         <b id="times_h" class="time"> </b>时
         <b id="times_m" class="time"> </b>分
         <b id="times_s" class="time"> </b>秒
     </div>
 </div>

 var time_wrap = document.getElementById("times_wrap");
     var time_d = document.getElementById("times_d");
     var time_h = document.getElementById("times_h");
     var time_m = document.getElementById("times_m");
     var time_s = document.getElementById("times_s");

     var time_end = new Date("2015/9/30 18:00:00");  // 设定结束时间
     time_end = time_end.getTime();

     function show_time(){
         var time_now = new Date();  // 获取当前时间
         time_now = time_now.getTime();
         var time_distance = time_end - time_now;  // 结束时间减去当前时间
         var int_day, int_hour, int_minute, int_second;
         if(time_distance >= 0){
             // 天时分秒换算
             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)

             // 时分秒为单数时、前面加零站位
             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;

             // 显示时间
             time_d.innerHTML = int_day;
             time_h.innerHTML = int_hour;
             time_m.innerHTML = int_minute;
             time_s.innerHTML = int_second;
         }else{
             time_d.innerHTML = time_d.innerHTML;
             time_h.innerHTML = time_h.innerHTML;
             time_m.innerHTML = time_m.innerHTML;
             time_s.innerHTML = time_s.innerHTML;
         }
     };
     window.setInterval(function(){
         show_time();
     }, 1000);