Ajax Calendar Extender tip NOT showing with a textbox

Despite downloading,installing and adding AjaxControlToolkit in VS2012(Ultimate) to my project, I am unable to access its "CalanderExtender" object feature.

.aspx file code is:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Documents.aspx.cs" Inherits="a.Documents" %><%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajaxToolkit"%><!DOCTYPE html><html xmlns="http://www.w3.org/1999/xhtml"><head runat="server"><title></title></head><body><form id="form1" runat="server"><div><asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager><asp:TextBox ID="TextBox1" runat="server"></asp:TextBox></div></form></body></html>

Web.config code is:

<?xml version="1.0" encoding="utf-8"?><!--
  For more information on how to configure your ASP.NET application, please visit
  http://go.microsoft.com/fwlink/?LinkId=169433
  --><configuration><system.web><compilation debug="true" targetFramework="4.5" /><httpRuntime targetFramework="4.5" /><pages><controls><add tagPrefix="ajaxToolkit" assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" /></controls></pages></system.web></configuration>

Now, what I don't understand is, despite adding

AjaxControlToolkit

into the project, adding

ScriptManager

from

AJAX Extensions

, adding and registering Assembly, CalanderExtender tip is not showing with the text box that I have added in the above code. When I try to manually add CalanderExtender using following code:

<asp:CalendarExtender ID="CalendarExtender1" runat="server"PopupButtonID="txtDate"TargetControlID="TextBox1"PopupPosition="Right"></asp:CalendarExtender>

It throws the following error in .aspx file:

ErrorCreatingControl-CalendarExtender1Unknown server tag 'asp:CalendarExtender'.

Irony of the fact is I have implemented this Ajax Calendar feature heaps of times before but never faced an issue as this one.

Please help.

Kind regards.