selcted date should not greater than today date is not working

hi to all thanks in advance ..compare validator is not working .please tell me how to do it
 
<asp:TextBox ID="txtPatientBirthDate" runat="server" CssClass="DialogTextBox"
                                                                        ValidationGroup="EditStudyValidationGroup"></asp:TextBox>
 
<asp:CompareValidator runat="server" ErrorMessage="must be greater than today in MM/DD/YYYY format"
                 &nb ...