using web methods with master pages

I've got a function on all pages in my site which is located in my master page and I want it to run from some jQuery Ajax method.

I've got some code like this at the moment

jQuery

$(document).ready(function() {
  $("#test").click(function() {
    $.ajax({
      type: "POST",
      url: "Default.aspx/GetDate",
      data: "{}",
      contentType: "application/json; charset=utf-8",
      dataType: "json",
      success: function(msg) {
        $("#test").text(msg.d);
      }
    });
  });
});

HTML in Master page

<div id="test">Click here for the time.</div>

Asp.Net code behind in my Master VB

<WebMethod> _
Public Shared Function GetDate() As String
    Return DateTime.Now.ToString()
End Function

Currently this doesn't work unless I move the web method to the Default.aspx VB page

Is there a way I can change this part

url: "Default.aspx/GetDate",

To use the master page function?

I tried changing it to

url: "template.master/GetDate",

But this just gives me a 404 error

Any ideas?

Thanks in advance