How do I compare two date fields in asp.net?

I have written the attached code for two textbox controls (OpenDateInsertTextBox, ClosingDateInsertTextBox) attached to ajax calendar extenders to compare each other so that the ClosingDate is never before the OpenDate. Unfortunately the compare only works when the OpenDates day is less than the ClosingDates day, i.e. OpenDate: 15/07/2009 ClosingDate: 13/08/2009 flags an error?

<td class="item">
                                                                <asp:TextBox ID="OpenDateInsertTextBox" runat="server" CssClass="calendarTxtBox"></asp:TextBox>
                                                                
                                                                <asp:RequiredFieldValidator ID="OpenDateValidator" runat="server" ControlToValidate="OpenDateInsertTextBox" ErrorMessage="*" SetFocusOnError="true" Display="Dynamic" ValidationGroup="insert"></asp:RequiredFieldValidator>
                                                                <asp:RegularExpressionValidator ID="OpenDateRegularValidator" runat="server" ErrorMessage="<br><small>Invalid date.</small>" ValidationExpression="^(?=\d)(?:(?:31(?!.(?:0?[2469]|11))|(?:30|29)(?!.0?2)|29(?=.0?2.(?:(?:(?:1[6-9]|[2-9]\d)?(?:0[48]|[2468][048]|[13579][26])|(?:(?:16|[2468][048]|[3579][26])00)))(?:\x20|$))|(?:2[0-8]|1\d|0?[1-9]))([-./])(?:1[012]|0?[1-9])\1(?:1[6-9]|[2-9]\d)?\d\d(?:(?=\x20\d)\x20|$))?(((0?[1-9]|1[012])(:[0-5]\d){0,2}(\x20[AP]M))|([01]\d|2[0-3])(:[0-5]\d){1,2})?$" Display="Dynamic" ControlToValidate="OpenDateInsertTextBox" ValidationGroup="insert"></asp:RegularExpressionValidator>
                                                                <ajaxToolKit:CalendarExtender ID="OpenDateCalendar" runat="server" TargetControlID="OpenDateInsertTextBox" Format="dd/MM/yyyy" CssClass="MyCalendar" PopupButtonID="OpenDateCalendarImage">
                                                                </ajaxToolKit:CalendarExtender>
                                                            </td>
                                                            <td class="item">
                                                                <br />
                                                                <asp:TextBox ID="ClosingDateInsertTextBox" runat="server" CssClass="calendarTxtBox"></asp:TextBox>
                                                                
                                                                <asp:RequiredFieldValidator ID="ClosingDateValidator" runat="server" ControlToValidate="ClosingDateInsertTextBox" ErrorMessage="*" SetFocusOnError="true" Display="Dynamic" ValidationGroup="insert"></asp:RequiredFieldValidator>
                                                                <asp:RegularExpressionValidator ID="ClosingDateRegularValidator" runat="server" ErrorMessage="<br><small>Invalid date.</small>" ValidationExpression="^(?=\d)(?:(?:31(?!.(?:0?[2469]|11))|(?:30|29)(?!.0?2)|29(?=.0?2.(?:(?:(?:1[6-9]|[2-9]\d)?(?:0[48]|[2468][048]|[13579][26])|(?:(?:16|[2468][048]|[3579][26])00)))(?:\x20|$))|(?:2[0-8]|1\d|0?[1-9]))([-./])(?:1[012]|0?[1-9])\1(?:1[6-9]|[2-9]\d)?\d\d(?:(?=\x20\d)\x20|$))?(((0?[1-9]|1[012])(:[0-5]\d){0,2}(\x20[AP]M))|([01]\d|2[0-3])(:[0-5]\d){1,2})?$" Display="Dynamic" ControlToValidate="ClosingDateInsertTextBox" ValidationGroup="insert"></asp:RegularExpressionValidator>
                                                                <asp:CompareValidator 
                                                                    ID="DateValidator" 
                                                                    runat="server" 
                                                                    ControlToCompare="OpenDateInsertTextBox" 
                                                                    ControlToValidate="ClosingDateInsertTextBox" 
                                                                    ErrorMessage="<br><small>Invalid date.</small>" 
                                                                    Operator="GreaterThan" 
                                                                    ValidationGroup="insert">
                                                                </asp:CompareValidator>
                                                                <ajaxToolKit:CalendarExtender ID="ClosingDateCalendar" runat="server" TargetControlID="ClosingDateInsertTextBox" Format="dd/MM/yyyy" CssClass="MyCalendar" PopupButtonID="ClosingDateCalendarImage">
                                                                </ajaxToolKit:CalendarExtender>
                                                            </td>

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