https://jsfiddle.net/5og49Lah/1/
<!DOCTYPE html>
<html>
<head>
<title>Page Title</title>
</head>
<body>
<div id="showtime"></div>
<h1>My First Heading</h1>
<p>My first paragraph.</p>
<script>
var showtime = function () {
var nowtime = new Date(), //获取当前时间
endtime = new Date("2022/8/31"); //定义结束时间
var lefttime = endtime.getTime() - nowtime.getTime(), //距离结束时间的毫秒数
leftd = Math.floor(lefttime/(1000*60*60*24)), //计算天数
lefth = Math.floor(lefttime/(1000*60*60)%24), //计算小时数
leftm = Math.floor(lefttime/(1000*60)%60), //计算分钟数
lefts = Math.floor(lefttime/1000%60); //计算秒数
return "距离本次活动结束还剩"+leftd + "天" + lefth + "时" + leftm + "分" + lefts + "秒"; //返回倒计时的字符串
}
var div = document.getElementById("showtime");
setInterval (function () {
div.innerHTML = showtime();
}, 1000); //反复执行函数本身
</script>
</body>
</html>