How to calculate due date in x++?

////      for aging report

class Duedate_CustAging

{

    public static void main(Args _args)

    {

        CustTrans               CustTrans;

        CustInvoiceJour         custInvoiceJour;

        ttsBegin;

        while select forupdate custInvoiceJour where CustInvoiceJour.Payment =='Test'

                        && CustInvoiceJour.DueDate == CustInvoiceJour.InvoiceDate

                          //  && CustTrans.Invoice=='INV-0000001623'

        {

            CustInvoiceJour.DueDate = PaymTerm::find(CustInvoiceJour.Payment).due(CustInvoiceJour.InvoiceDate);

            CustInvoiceJour.update();

        }

        ttsCommit;

    }

}

////      for Cust Trans

class DuedateCusttrans_OpenClass

{

    public static void main(Args _args)

    {

        CustTransOpen       CustTransOpen;

        CustTrans           CustTrans;

        

        ttsBegin;

        while select forupdate CustTransOpen join CustTrans where CustTrans.RecId == CustTransOpen.RefRecId 

                    && CustTrans.AccountNum == CustTransOpen.AccountNum 

                    && CustTrans.PAYMTERMID ==CustParameters::find().CustPaymTermId

                    && CustTransOpen.DueDate == CustTransOpen.TransDate  && CustTrans.Invoice !=''

                    //  && CustTrans.Invoice=='INV-0000001623'

        {

            CustTransOpen.DueDate = PaymTerm::find(CustTrans.PAYMTERMID).due(CustTransOpen.TransDate);

            CustTransOpen.update();

        }

        

        ttsCommit;

        

        info('Due date has been Updated');

    }

}