def factor(base = nil)
      return AVERAGE_FACTOR if base.nil?
      return calculation(1, base) if atom.zero?

      calculation(atom, base)
    end
def factor(base = nil)
      return AVERAGE_FACTOR if base.nil?
      return calculation(1, base) if atom.zero?

      calculation(atom, base)
    end
@param [Numeric] atom The atom value

 The Month factor

 @param [ISO8601::DateTime, nil] base (nil) The base datetime to compute
   the month length.

 @return [Numeric]