Date Validation for Ajaxcalendar extendar control in asp.net

 <script language="javascript" type="text/javascript">

        function CheckEndDate(sender, args) {

            date1 = new Date()

            date1.setHours(0, 0, 0, 0);

            if (sender._selectedDate < date1) {

                alert("Expiry Date should not be less than current date! ");

                sender._selectedDate = new Date();

                // set the date back to the today

                sender._textbox.set_Value(sender._selectedDate.format(sender._format))

            }

            else {

                

            }

        }

    </script>

 <asp:TextBox ID="txtStartDate" runat="server"> </asp:TextBox>

                                            <asp:ImageButton ID="CalendarImage1" runat="server" Height="18px" ImageAlign="AbsMiddle" ImageUrl="~/App_Themes/Default/images/Calendar_view_month.png" />

                                            <cc1:CalendarExtender ID="CalendarExtender1" runat="server" Enabled="True" FirstDayOfWeek="Sunday" Format="dd/MM/yyyy" PopupButtonID="CalendarImage1" TargetControlID="txtStartDate">

 </cc1:CalendarExtender>