倒计时案例```js```主要应用到日期对象。new Date("yyyy-mm-dd hh:mm:ss")。

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>倒计时</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        div {
            width: 60px;
            height: 20px;
            padding: 20px 0;
            border: 1px solid #000;
            background-color: rgb(226, 197, 178);
            display: inline-block;
            text-align: center;
        }
    </style>
</head>

<body>
    <div class="hour"></div>
    <div class="minute"></div>
    <div class="second"></div>
    <script>
        var hour = document.querySelector(".hour");
        var minute = document.querySelector(".minute");
        var second = document.querySelector(".second");

        var inputTime = +new Date('2021-11-11 00:00:00');//返回用户输入时间总毫秒数
        countDown();  //先调用一次函数，防止刷新时出现空白
        setInterval(countDown,1000) //开启定时器
        function countDown() {
            var nowTime = +new Date();//返回的是当前时间总的毫秒数
            var times = (inputTime - nowTime) / 1000;  //times为剩余时间总的秒数
            var h = parseInt(times / 60 / 60 % 24);  //时
            h = h < 10 ? '0' + h : h;
            hour.innerHTML=h; //把剩余的小时给小时的盒子
            var m = parseInt(times / 60 %60 );  //分
            m = m < 10 ? '0' + m : m;
            minute.innerHTML=m; 
            var s = parseInt(times % 60 );  //秒
            s = s < 10 ? '0' + s : s;
            second.innerHTML=s; 
        }
    </script>
</body>

</html>