How to set Startdate is Today in cc2:CalendarExtender?

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" 
                                                             TagPrefix="cc2" %>
<asp:TextBox ID="txtngaydat" runat="server"></asp:TextBox>
<cc2:CalendarExtender ID="CalNgayDat" TargetControlID="txtngaydat" 
                       PopupButtonID="ibtCal" runat="server"></cc2:CalendarExtender>

Any help would be highly appreciated, Thanks in advance,