function showTime() {
//时间
nowtime = new Date();
year = nowtime.getFullYear();
month = nowtime.getMonth() + 1;
date = nowtime.getDate();
document.getElementById("mytime").innerHTML = year + "年" + month + "月" + date + "日" + nowtime.toLocaleTimeString();
}
setInterval("showTime()", 1000);
<span id="mytime">__年_月_日</span>
原文：https://www.cnblogs.com/songjiabo/p/9071466.html