How to use AJAX calendar extender and Limit it to not select Future Dates ?

Step1:  Add Ajax Toolkit

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>

<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"></asp:ToolkitScriptManager>

Step2: Add Calendar Extender along with Control ID

<asp:CalendarExtender ID="CalendarExtender" runat="server" TargetControlID="txtDateTime"></asp:CalendarExtender>

<asp:TextBox ID="txtDateTime" runat="server" AutoPostBack="True"></asp:TextBox>

Step3: Disable Future Dates

  protected void Page_Load(object sender, EventArgs e)
    {

        CalendarExtender.EndDate = DateTime.Now.AddDays(-1);
    }