Textchanged event is not firing wuth the help of ajax.

When I change the text it should calculate the days on textchanged event but it won't work!
Design:

<asp:ScriptManager ID="ScriptManager1" runat="server">
       </asp:ScriptManager>
       <asp:UpdatePanel ID="UpdatePanel2" runat="server">
    
       <ContentTemplate>
       <asp:TextBox ID="txtbxfrm" runat="server" CssClass="textbox"
           ViewStateMode="Enabled" ontextchanged="txtbxfrm_TextChanged"
               AutoPostBack="True"></asp:TextBox>
       &nbsp;&nbsp;<asp:Label ID="lblntc" runat="server" CssClass="validate"
         ...