例子

1.给定一个时间，然后写出它和当前时间的倒计时（从毫秒开始转换）

<p id="app">
        00:00:00;
    </p>
    <script>
        function handleTime(value) {
            if (value < 10) {
                return "0" + value;
            }
            return "" + value;
        }
        function showTime() {
            var app = document.getElementById("app");
            var endTime = "2020-12-01 23:50:00";
            //1.获取结束时间的毫秒
            endTime = new Date(endTime).getTime();
            //2.获取当前时间的毫秒
            var nowTime = new Date().getTime();
            //3.获取他们之间的差
            var gapTime = endTime - nowTime;
            console.log(gapTime);
            //4.毫秒和时分秒的转换
            var s = Math.floor(gapTime / 1000);
            var houer = handleTime(Math.floor(s / 3600));
            s = Math.floor(s % 3600);
            var minute = handleTime(Math.floor(s / 60));
            second = handleTime(Math.floor(s % 60));
            console.log(houer);
            console.log(minute);
            console.log(second);
            var sum = houer + ":" + minute + ":" + second;
            console.log(sum);
            app.innerHTML=sum;
          // setTimeout(function(){
            //     showTime()
            // },1000)
        }
         //showTime();
        showTime();
        setInterval(function(){
            showTime();
        },1000)
  </script>     //函数的两种写法

2.动态时钟

 <div>
        <img src="./images/0.png" alt="">
        <img src="images/0.png"  alt="">
        <span>:</span>
        <img src="images/0.png"  alt="">
        <img src="images/0.png"  alt="">
        <span>:</span>
        <img src="images/0.png"  alt="">
        <img src="images/0.png"  alt="">
    </div>
    <script>
        function handleTime(value) {
                if (value < 10) {
                    return "0" + value;
                }
                return "" + value;
            }
        function showTime() {
            var d = new Date();
            var hour = handleTime(d.getHours());
            var minute = handleTime(d.getMinutes());
            var second = handleTime(d.getSeconds());
            var imgs = document.querySelectorAll("img");
            var arr = hour + minute + second;
            // console.log(arr);
            //将图片的地址设置为动态
            for (i = 0; i < imgs.length; i++) {
                imgs[i] .src= "images/" + arr[i] + ".png"
               //imgs[i].src=`images/`${sum[i]}.png`;
            }
            setTimeout(function(){
                showTime()
            },1000)
         }
         showTime();
    </script>