//获取服务器时间
var getServerDate = function () {
    var xmlHttpRequest = null,
    serverDate = new Date();
    if (window.ActiveXObject) {
         xmlHttpRequest = new ActiveXObject("Microsoft.XMLHTTP");
    }
    else if (window.XMLHttpRequest) {
         xmlHttpRequest = new XMLHttpRequest();
    }
    else {
         xmlHttpRequest = new ActiveXObject("Msxml2.XMLHTTP");
    }
    //同步请求 HEAD请求仅获取HEAD信息
    xmlHttpRequest.open('HEAD', '/', false);
    xmlHttpRequest.send(null);
    if (xmlHttpRequest.status === 200) {
        // 获取请求响应头里的时间戳
        serverDate = new Date(xmlHttpRequest.getResponseHeader("Date"));
    }
    return serverDate;
}