c# date compare validation

i have two text vox with calender image
one is start date another is end date i want end date aways greater than start date how i validate plz help me
<asp:CompareValidator ID="CompareValidator1" 
                                                                ControlToValidate="txt_enddate" Type="Date" Operator="GreaterThanEqual"
                                                                ErrorMessage="end date should be greated than startdate"
                                                                ControlToCompare="start_sdate" runat="server"></asp:CompareValidator>