<%@ 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>
I think you need to set StartDate on Page_Load event.
protected void Page_Load(object sender, EventArgs e)
{
CalNgayDat.StartDate = DateTime.Now;
}