添加到主题选项的页脚代码，或者复制如下代码到 footer.php 页脚那里

<span id="runtime_span"></span><script type="text/javascript">function show_runtime(){window.setTimeout("show_runtime()",1000);X=new Date("12/01/2021 1:21:40");Y=new Date();T=(Y.getTime()-X.getTime());M=24*60*60*1000;a=T/M;A=Math.floor(a);b=(a-A)*24;B=Math.floor(b);c=(b-B)*60;C=Math.floor((b-B)*60);D=Math.floor((c-C)*60);runtime_span.innerHTML="本站已运行"+A+"天"+B+"小时"+C+"分"+D+"秒"}show_runtime();</script>