实现倒计时效果

         //<div id='date'><div>
        var end = new Date('2021-1-1 00:00');
        function countDone(){
            var now = new Date();
            var remainder = end - now;
            if(remainder<0){
                return;
            }
            var day = Math.floor(remainder/1000/60/60/24); //天
            var hour = Math.floor(remainder/1000/60/60%24); //时
            var minute = Math.floor(remainder/1000/60%60); //分
            var second = Math.floor(remainder/1000%60); //天
            var str = `还有${day}天${hour}时${minute}分${second}秒`;
            var date = document.getElementById('date');
            date.innerHTML = str;
            setTimeout(countDone,1000)
        }
        countDone()

利用两个日期对象，相减得到时间间隔的毫秒数，然后进行加工处理。