=IFERROR(Recurs@row = "Monthly", (DATE(YEAR(Due@row), MONTH(Due@row) + 1, DAY(Due@row))),IFERROR(Recurs@row="Annually",(DATE(YEAR(Due@row)+1, MONTH(Due@row), DAY(Due@row)))))
Works like a charm until a date in December would happen in January, then it throws an error.