简单倒计时

        <div id="box"></div>
        <script>
            var box = document.getElementById("box");
            var span = document.getElementById("span");
            var end,now,allTime,days,hours,minutes,seconds,millise,time;
            time = "2017-9-19 13:21:00:00";
            end = new Date(time);
            function curstor(){
                now = new Date();
                allTime = end.getTime()-now.getTime();
                days =  Math.floor(allTime/1000/60/60/24);//求出整的天数
                hours = Math.floor((allTime/1000/60/60)%24)//求出所有的小时数，然后取模求出不足一天的小时数;
                minutes = Math.floor((allTime/1000/60)%60);//求出整的分钟数,然后取模求出不足一小时的分钟数
                seconds = Math.floor((allTime/1000)%60);//求出整的秒数，然后取模求出不足一分钟的秒数
                millise = Math.floor((allTime)%60);//求出所有的秒数，然后取模求出不足一秒的毫秒数;
                if(hours<10){
                    hours="0"+hours;
                }
                if(minutes<10){
                    minutes="0"+minutes;
                }
                if(seconds<10){
                    seconds="0"+seconds;
                }
                if(millise<10){
                    millise="0"+millise;
                }
                box.innerHTML="距离"+time+"还剩下："+days+"天"+hours+"小时"+minutes+"分钟"+seconds+"秒"+millise+"毫秒";
                // console.log(allTime);
                if(allTime<0){
                    clearInterval(box.t);
                    days=0;
                    hours=0;
                    minutes=0;
                    seconds=0;
                    millise=0;
                    box.innerHTML="距离"+time+"还剩下："+days+"天0"+hours+"小时0"+minutes+"分钟0"+seconds+"秒0"+millise+"毫秒";
                }
            }
            // curstor();
            box.t = setInterval("curstor()",0);

        </script>