What does {"d":""} means in asp.net webservice response

I've created a simple C# asp.net web service function which returns a string message
and I am calling it from page using jquery ajax.

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);
            }
        });
    });

My question is that why does alert(msg); doesnt works