Ho to calculate due date in x++?

 class UpdateDuedate_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 aging report

class UpdateDuedateCusttrans_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');

      

    }

}