Access Header Info in requestpage

Hello, I am trying to display a simple message to the user to double check their posting date if it's older than 2 days when printing the report.  I cannot seem to get access to the Posting Date from the Sales Header in order to do the date comparison.  I have tried many different ways, this is where my code currently stands.

field(PackSlip; PackSlip)
                        ApplicationArea = Basic, Suite;
                        Caption = 'Generate Packing Slip';
                        ToolTip = 'Generates a packing slip with no pricing or scheduled ship date.';
                        trigger OnValidate()
                            today: date;
                            date_lim: date;

                            today := today();
                            date_lim := calcdate('-2D', today);

                            if PackSlip then begin
                                BlankShipDate_Enabled := True;
                                message('today ' + format(today, 0, '<Month,2>/<Day,2>/<Year4>'));
                                message('date_lim ' + format(date_lim, 0, '<Month,2>/<Day,2>/<Year4>'));
                                message('post date ' + format(post_date, 0, '<Month,2>/<Day,2>/<Year4>'));
                                message('post date #2 ' + format(SalesHeader."Posting Date", 0, '<Month,2>/<Day,2>/<Year4>'));

                                if SalesHeader."Posting Date" <= date_lim then
                                    message('Your Posting Date is older than 2 days, please ensure your Posting Date is set to date of shipment before printing!');
                            else begin
                                BlankShipDate_Enabled := False;
                                ShipDate_Enabled := False;
                                BlankShipDate := False;