use this no problem at all.
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<asp:TextBox ID="txtStartDate" runat="server" CssClass="textbox"></asp:TextBox><code></code>
<asp:ImageButton ID="ibtnSatrtDate" runat="server" Height="16px" Width="16px" ImageUrl="~/images/Calendar.png" />
<cc1:CalendarExtender ID="txtStartDate_CalendarExtender" runat="server" Enabled="True"
TargetControlID="txtStartDate" PopupButtonID="ibtnSatrtDate">
</cc1:CalendarExtender>