How to get time elapsed after loading a webpage

How to get the time elapsed time after loading a webpage, using Callback() in MS Ajax library. Here is a sample script;

[Ajax-Script]

   1: <script type="text/javascript">

   2:         // Page Load

   3:         function pageLoad() {

   4:             var loadTime = {date : new Date()};

   5:             var callback = Function.createCallback(onBtnClick, loadTime);

   6:             

   7:             $addHandler($get("Button1"), "click", callback);

   8:         }

   9:  

  10:         function onBtnClick(event, context) {

  11:             var loadTime = context.date;

  12:             var elapseTime = (new Date() - loadTime) / 1000;

  13:  

  14:             $get("Text1").value = elapseTime + " Seconds";

  15:         }

  16:     </script>

[HTML-Script]

   1: <form id="form1" runat="server">

   2:     <asp:ScriptManager ID="ScriptManager1" runat="server">

   3:     </asp:ScriptManager>

   4:     <div>

   5:         Time elapsed since PageLoad: 

   6:         <input id="Text1" type="text" />

   7:         <input id="Button1" type="button" value="Get Elapsed Time" />

   8:     </div>

   9:     </form>

Thanks.