how update documentdate after posting PO

Hi, I'm trying to posting a packing slip, and everything goes fine, but I have a little problem with dates, I've changed parameters date, I send invoice date instead posting date, the problem is when we check vendor´s transaction the transdate takes invoice date and document date is blank, I need to send both dates (posting and document) to fill all fields correctly, and I repeat, posting goes fine,  Just I need to know how to send  document and posting dates, Somebody could help me, please , here is my code:

purchTable = PurchTable::find(OrderNum);

        purchFormLetter = purchFormLetter::construct(DocumentStatus::Invoice);
        purchFormLetter.reArrangeNow(false);
        purchFormLetter.GsThreeWayMatch(true);
        purchFormLetter.update(purchTable,
                               _num,
                               _gsaccpayxmlheader.Date, // systemDateGet(), ****Here I send document date or posting but need both
                               PurchUpdate::PackingSlip,
                               AccountOrder::None,
                               purchFormLetter.proforma(),
                               purchFormLetter.printFormLetter(),
                               purchFormLetter.usePrintManagement(),
                               purchFormLetter.creditRemaining(),
                               listRecord.pack());