网站如何加入显示运行时间

<span id="runtime_span"></span>
<script type="text/javascript">
function show_runtime()
{window.setTimeout("show_runtime()",1000);
X=new Date("10/16/2017 5:22:00");
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="本站勉强运行: "+A+"天"+B+"小时"+C+"分"+D+"秒"}show_runtime(); 
</script> Date("10/16/2017 5:22:00")