calendar control in asp.net

<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="asp" %>






<tr>
            <td class="style2">
                Contract Start Date</td>
                                            <td>
                                                <asp:TextBox ID="TextBox17" runat="server" Width="175px"></asp:TextBox>
                                                <asp:ImageButton ID="ImageButton1" runat="server" Width="16px"
                                                    ImageUrl="~/images/Calendar.png" onclick="ImageButton1_Click" />
                                              
                                                
                                                <asp:Panel ID="Panel2" runat="server">
                                                 <asp:Calendar ID="Calendar1" runat="server" Width="80px" 
          onselectionchanged="Calendar1_SelectionChanged" ></asp:Calendar>

                                                </asp:Panel>
                                                                                              
                                                <asp:RequiredFieldValidator ID="RequiredFieldValidator17" runat="server"
                        ControlToValidate="TextBox17" ErrorMessage="Please Fill Contract Start Date" Display="Dynamic"
                                                    ValidationGroup="1"></asp:RequiredFieldValidator>
                                                 <br />
        <asp:CalendarExtender ID="calendarButtonExtender" runat="server" TargetControlID="TextBox17"
            PopupButtonID="Image1" />
                                            </td>
                                        </tr>