什么{“d”：“”}表示在asp.net webservice响应中

C#：

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public string HelloWorld() {
    return DateTime.Now.ToString();
}

JS：

$(document).ready(function() {
    //alert("ready");
        $.ajax({
            type: "POST",contentType: "application/json; chatset=utf-8",url: "WebService2.asmx/HelloWorld",data: "{}",dataType: "json",success: function(msg) {
                //alert(msg); //doesnt works
                alert(msg.d);
            }
        });
    });

我的问题是为什么警报(msg);不起作用