Asp.net Ajax textbox calendar extender updating but only on screen

Question

The code is the one below. I've tried several solution from other posts but none worked. This should be a very simple task tough. After clicking on the image, the calendar appears, the textbox is updated but when I get the .text property it still is unchanged. Thanx for any help

<%@ Page Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true" Inherits="Comunicazioni_ClassificaInterna" Title="Classifica interna" Codebehind="ClassificaInterna.aspx.cs" %>
<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="cc1" %>
<asp:Content ID="Content1" ContentPlaceHolderID="MainContent" runat="server">



    <asp:ScriptManager ID="ScriptManager1" runat="server" EnableScriptGlobalization="True" EnableScriptLocalization="False"> </asp:ScriptManager>

    <br />



    <asp:TextBox ID="tbCompetitionDate" runat="server" ReadOnly="True"></asp:TextBox>
    <cc1:CalendarExtender ID="tbCompetitionDate_CalendarExtender" runat="server" PopupButtonID="imgPopup" Format="dd/MM/yyyy" BehaviorID="tbCompetitionDate_CalendarExtender" TargetControlID="tbCompetitionDate" FirstDayOfWeek="Monday" />
    <asp:ImageButton ID="imgPopup" ImageUrl="../../../Resources/Pics/calendar.png" ImageAlign="Bottom" runat="server" Height="25px" Width="30px" />

    <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />

</asp:Content>


<asp:Content ID="Content3" ContentPlaceHolderID="FooterPlaceHolder" runat="server">      
        <img alt="" src="../../Resources/Pics/Footer.png"  style="margin-left:500px; margin-top:200px;   width:300px;height:50px" />
    </asp:Content>