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;

    }

}