用到Date对象和BOM中的setInterval()定时器。
效果如下（每隔1s刷新一次）
代码
<!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>BOM</title><style></style>
</head>
<body><div>中国标准时间<p>14:08:00</p></div>
</body>
<script>function update() {var p = document.querySelector('p');var date = new Date();console.log(date);var h = date.getHours();h < 10 ? h = "0" + h : h;var m = date.getMinutes();m < 10 ? m = "0" + m : m;var s = date.getSeconds();s < 10 ? s = "0" + s : s;p.innerHTML = h + ":" + m + ":" + s;}setInterval(update, 1000);</script>
</html>
