Ajax Controls

<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager><asp:CalendarExtender ID="CalendarExtender1" TargetControlID="txtFittingDate" runat="server" /><asp:TextBox ID="txtFittingDate" runat="server" Width="80px"></asp:TextBox>

Hi folks, I have Ajax controls on my tool bar, I’m using visual Studio 2012, when I tried to insert the calendar picker, at runtime I am presented with the following error;

‘Calendar Extender’ is not a know element