How to disable current date

Question

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="CalendarUC.ascx.cs" Inherits="CalendarUC" %>

<%@ Register Namespace="AjaxControlToolkit" Assembly="AjaxControlToolkit" tagPrefix="cc1" %>
<table>
<tr>
<td>
<asp:TextBox ID="txtDate" runat="server" Width="95px"/>
<asp:ImageButton ID="imgbtnCalendar" runat="server"
  ImageUrl="~/Images/Calendar.png" BorderStyle="Solid" BorderWidth="1px"
  Height="20px" ImageAlign="Top" Width="25px" />
<cc1:CalendarExtender ID="CalendarExtender2" TargetControlID="txtDate"
PopupButtonID="imgbtnCalendar" Format="dd-MMM-yyyy" runat="server" />
</td>
</tr>
</table>

from my above code
i want to add code to disable current date.for that how to write the code from my above code please help m