1. new Date()

不设参数，则获取当前时间； 设参数，则获取参数内时间

要创建日期对象，就使用 new 操作符来调用 Date 构造函数：

let now = new Date();

2. 年,月,日,时,分,秒,毫秒

获取当前时间的年,月,日,时,分,秒,毫秒

<!-- 获取当前的时间 -->
    <script>
        // Date
        var d = new Date();
        /* 年,月,日 */
        var year = d.getFullYear();
        var month = d.getMonth();
        var date =  d.getDate();
        /* 时,分,秒,毫秒
        1s =1000 millisecond
         */
        var hour =  d.getHours();
        var minute = d.getMinutes()
        var second = d.getSeconds();
        var mills = d.getTime();
        console.log(d);//Tue Dec 01 2020 22:05:23 GMT+0800 (中国标准时间)
        console.log(year);
        console.log(month+1)   //月份是从0开始的,要加1
        console.log(date)
        console.log(hour)
        console.log(minute)
        console.log(second);
        console.log(mills)
    </script>

3. 倒计时时钟

<p id="app">
    10:10:01
</p>
<script>
    function handleTime(value) {
        if (value < 10) {
            return "0" + value;
        }
        return value;
    }
    function showTime() {
        var app = document.getElementById("app")
        var endTime = "2020-12-02 00:00:00";
        /* 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;
        /* 4.将他们的差转换为时分秒 */
        var seconds = Math.floor(gapTime / 1000);
        var hours = handleTime(Math.floor(seconds / 3600));
        seconds = seconds % 3600;
        var minutes = handleTime(Math.floor(seconds / 60));
        seconds = handleTime(seconds % 60);
        var sum = hours + ":" + minutes + ":" + seconds;
        app.innerHTML = sum;
        setTimeout(function(){
            showTime()
        },1000)
    }
    showTime();
</script>

或者

showTime();
setInterval(function(){
    showTime();
},1000)

4.动态时钟

<div id="app">
        <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 imgs = document.querySelectorAll("#app img")
            var d = new Date();
            var hours = handleTime(d.getHours());
            var minutes = handleTime(d.getMinutes());
            var seconds = handleTime(d.getSeconds());
            var sum = hours + minutes + seconds;
            console.log(sum);
            for (var i = 0; i < imgs.length; i++) {
                console.log(sum[i])
                // imgs[i].src = "images/" + sum[i] + ".png";
                imgs[i].src = `images/${sum[i]}.png`;
            }
            setTimeout(function(){
                showTime()
            },1000)
        }
        showTime()
    </script>