month.rb

require "date"

class Month
  include Comparable

  MONTHS_PER_YEAR = 12

  def self.from_date(date)
    self.from_parts(date.year, date.month)
  end

  def self.from_parts(year, month)
    months_since_jan = month - 1
    months_since_one_bce = (year * MONTHS_PER_YEAR) + months_since_jan

    new(months_since_one_bce)
  end

  def initialize(since_one_bce)
    # months since Jan, 1BCE (there is no year zero)
    # https://en.wikipedia.org/wiki/Year_zero
    @since_one_bce = since_one_bce
  end

  # Accessors
  # We need to use integer division and modulo to break down our single internal
  # value into different granularities for human consumption.
  def month
    # we want Jan to be month 1, not 0 to match the Ruby Date#month API
    (since_one_bce % MONTHS_PER_YEAR) + 1
  end

  def year
    (since_one_bce / MONTHS_PER_YEAR)
  end

  # <=> and succ required to work with ranges
  def <=>(other)
    since_one_bce <=> other.since_one_bce
  end

  def succ
    self.class.new(since_one_bce.succ)
  end

  # Since this is a value object, we want eql?, hash, and == to match when the
  # internal value of two month objects is the same. We get == for free from
  # Comparable but need to implement the others ourselves.
  def hash
    [self.class, since_one_bce].hash
  end

  alias_method :eql?, :==

  def inspect
    "#{Date::MONTHNAMES[month]} #{year}"
  end

  protected

  attr_reader :since_one_bce
end

month_spec.rb

require_relative "month"
require "date"

RSpec.describe Month do
  describe ".from_date" do
    it "correctly pulls the year and month" do
      date = Date.new(2020, 6)
      month = Month.from_date(date)

      expect(month.year).to eq 2020
      expect(month.month).to eq 6
    end

    it "treats january as the first month, not zero" do
      date = Date.new(2020, 1)
      month = Month.from_date(date)

      expect(month.month).to eq 1
    end
  end

  describe ".from_parts" do
    it "correctly pulls the year and month" do
      month = Month.from_parts(2020, 6)

      expect(month.year).to eq 2020
      expect(month.month).to eq 6
    end

    it "treats january as the first month, not zero" do
      month = Month.from_parts(2020, 1)

      expect(month.month).to eq 1
    end
  end

  describe "#<=>" do
    it "is 0 for another instance of the same value" do
      m1 = Month.from_parts(2020, 2)
      m2 = Month.from_parts(2020, 2)

      expect(m1 <=> m2).to eq 0
    end

    it "is -1 for another instance that is later" do
      m1 = Month.from_parts(2020, 2)
      m2 = Month.from_parts(2020, 3)

      expect(m1 <=> m2).to eq(-1)
    end

    it "is 1 for another instance that is earlier" do
      m1 = Month.from_parts(2020, 2)
      m2 = Month.from_parts(2020, 1)

      expect(m1 <=> m2).to eq 1
    end
  end

  describe "#succ" do
    it "generates the next month" do
      m1 = Month.from_parts(2020, 1)
      m2 = m1.succ

      expect(m2.year).to eq 2020
      expect(m2.month).to eq 2
    end

    it "correctly wraps around the year boundary" do
      m1 = Month.from_parts(2020, 12)
      m2 = m1.succ

      expect(m2.year).to eq 2021
      expect(m2.month).to eq 1
    end
  end

  describe "#hash" do
    it "is the same as another instance of the same value" do
      m1 = Month.from_parts(2020, 1)
      m2 = Month.from_parts(2020, 1)

      expect(m1.hash).to eq m2.hash
    end

    it "is the different from another instance of a different value" do
      m1 = Month.from_parts(2020, 1)
      m2 = Month.from_parts(2020, 2)

      expect(m1.hash).not_to eq m2.hash
    end
  end

  describe "#eql?" do
    it "two instances are equal if they have the same value" do
      m1 = Month.from_parts(2020, 1)
      m2 = Month.from_parts(2020, 1)

      expect(m1).to be_eql m2
    end

    it "two instances are not equal if they have a different value" do
      m1 = Month.from_parts(2020, 1)
      m2 = Month.from_parts(2020, 2)

      expect(m1).not_to be_eql m2
    end
  end

  describe "#==" do
    it "two instances are equal if they have the same value" do
      m1 = Month.from_parts(2020, 1)
      m2 = Month.from_parts(2020, 1)

      expect(m1).to eq m2
    end

    it "two instances are not equal if they have a different value" do
      m1 = Month.from_parts(2020, 1)
      m2 = Month.from_parts(2020, 2)

      expect(m1).not_to eq m2
    end
  end

  describe "#equal?" do
    it "is equal if two object share the same identity" do
      m1 = Month.from_parts(2020, 1)

      expect(m1).to be_equal m1
    end

    it "is not equal if two different objects share the same value" do
      m1 = Month.from_parts(2020, 1)
      m2 = Month.from_parts(2020, 1)

      expect(m1).not_to be_equal m2
    end
  end
end

Loved the blog post!

FYI, looks like the describe block name for from_parts is blank.

Good catch @mthadley! Fixed.