user this ..its simple
<![CDATA[<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>]]>
<asp:TextBox ID="txtStartDate" runat="server" CssClass="textbox"></asp:TextBox>
<asp:ImageButton ID="ibtnSatrtDate" runat="server" Height="16px" Width="16px" ImageUrl="~/images/Calendar.png" />
<cc1:CalendarExtender ID="txtStartDate_CalendarExtender" runat="server"
TargetControlID="txtStartDate" PopupButtonID="ibtnSatrtDate">
</cc1:CalendarExtender>