js实现倒计时效果

<h4 class="title">距离双十一，还有</h4>

    <div class="time-item">
        <span><span id="day">00</span>天</span>
        <strong><span id="hour">00</span>时</strong>
        <strong><span id="minute">00</span>分</strong>
        <strong><span id="second">00</span>秒</strong>
    </div>

//
<script>
        //目标时间的时间戳
        var endDate = new Date('2020-11-11 0:0:0');
        var end = endDate.getTime();

        //获取需要的元素
        var spanday = document.getElementById('day');
        var spanhour = document.getElementById('hour');
        var spanminute = document.getElementById('minute');
        var spansecond = document.getElementById('second');

        //开启定时器
        setInterval(countdown, 1000);
        countdown();
        //倒计时
        function countdown() {
            //当前时间戳
            var nowDate = new Date();
            var now = nowDate.getTime();
            //计算时间差(毫秒)
            var leftTime = end - now;
            //相差秒数
            leftTime /= 1000;

            // 求 相差的天数/小时数/分钟数/秒数
            var day, hour, minute, second;
			//计算时间
            day = Math.floor(leftTime / 60 / 60 / 24);
            hour = Math.floor(leftTime / 60 / 60 % 24);
            minute = Math.floor(leftTime / 60 % 60);
            second = Math.floor(leftTime % 60);

            //设置盒子里面的内容
            spanday.innerText = day;
            spanhour.innerText = hour;
            spanminute.innerText = minute;
            spansecond.innerText = second;
        }
    </script>

注意：里面我没有写css样式，如果你想效果好看一点，你可以自己加上一些样式上去。