获取服务器的时间

之前写了获取本地时间的代码，后来发现很不仁道，如果别人的机器时间不对，页面上显示的时间也不对了，会给别人造成困扰。于是乎将代码改了一下，用xmlHttpRequest对象来取得服务器上的时间

<script language="JavaScript" type="text/JavaScript">
var myday = "";
var mymonth = "";
var myweekday = "";
var year = "";
var myyear = "";
var weekday = "";
var http_request = "";
if(window.XMLHttpRequest){
  try{http_request = new XMLHttpRequest();} catch (e){;}  
}else if(window.ActiveXObject){ 
  try{ http_request = new ActiveXObject("Msxml2.XMLHTTP");} catch (e){ ; }
  if( http_request == null){
    try { http_request = new ActiveXObject("Microsoft.XMLHTTP");}   catch (e){;}
  }
}
http_request.open('HEAD', '.', false);
http_request.send(null);
var mydate = new Date(http_request.getResponseHeader('Date'));
myweekday=mydate.getDay();
mymonth=mydate.getMonth()+1;
myday= mydate.getDate();
myyear= mydate.getFullYear();
if(myweekday == 0)
weekday=" 星期日 ";
else if(myweekday == 1)
weekday=" 星期一 ";
else if(myweekday == 2)
weekday=" 星期二 ";
else if(myweekday == 3)
weekday=" 星期三 ";
else if(myweekday == 4)
weekday=" 星期四 ";
else if(myweekday == 5)
weekday=" 星期五 ";
else if(myweekday == 6)
weekday=" 星期六 ";
document.write(year+"年"+mymonth+"月"+myday+"日&nbsp;"+weekday);
</script> 

获取服务器端的时间，怎么做到每一时刻都和服务器端时间同步？
不停的和服务器交互来获取？