//从response header中获取服务器当前时间,不存在有缓存时的问题
function getServerTime(){
     var xmlHttp = false;
     try {
       xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
     } catch (e) {
       try {
          xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
       } catch (e2) {
          xmlHttp = false;
       }
     }
    
     if (!xmlHttp && typeof XMLHttpRequest != 'undefined') {
       xmlHttp = new XMLHttpRequest();
     }
    
     xmlHttp.open("GET",window.location.href.toString(),false);
     xmlHttp.setRequestHeader("Range","bytes=-1");
     xmlHttp.send(null);
    
     var severtime=new Date(xmlHttp.getResponseHeader("Date"));
     return severtime
}