javascript 实时获得服务器上时间

gistfile1.js

var http = new XMLHttpRequest;
http.open("HEAD", ".", false);
http.send(null);
var curDate = new Date;
var offsetTime = curDate - Date.parse(http.getResponseHeader("Date"));
setInterval(function()
{
    curDate.setTime(new Date - offsetTime);
    document.getElementById("spnTime").innerHTML = curDate.toLocaleString();
}, 1000);