public Library CheckOutBook(Book book, Student student, DateTime checkedOutDate)
 {
     var result = new Library();
     result.ExpectedReturnDate = checkedOutDate.AddMonths(1);
     result.CheckOutDate = checkedOutDate;
     return result;
 }
        public Library CheckInBook(Book book, DateTime droppedOffDate)
        {
            var result = new Library();
            result.ExpectedReturnDate = null;
            result.ActualReturnDate = droppedOffDate;

                return result;
        }