Ruby Month class.

month.rb

require "attr_extras"  # gem

class Month
  vattr_initialize :year, :month_number

  def self.from(object)
    case object
    when Month then object
    else new(object.year, object.month)
    end
  end

  # Relies on Rails' Time.zone.
  def self.now
    from(Time.zone.now)
  end

  def first_date
    Date.new(year, month_number, 1)
  end

  def last_date
    Date.new(year, month_number, -1)
  end

  def last_day
    last_date.day
  end
  
  def date_for_day(number)
    Date.new(year, month_number, number)
  end

  def previous
    self.class.from(first_date - 1)
  end

  def next
    self.class.from(last_date + 1)
  end
end

month_spec.rb

require "spec_helper"

describe Month do
  it "has value object equality" do
    expect(Month.new(2014, 6)).to eq Month.new(2014, 6)
  end
end

describe Month, ".from" do
  it "builds a Month from a Date" do
    month = Month.from(Date.new(2014, 6, 1))
    expect(month).to eq Month.new(2014, 6)
  end

  it "builds a Month from a Time" do
    month = Month.from(Time.zone.local(2014, 6, 1,  0, 0, 0))
    expect(month).to eq Month.new(2014, 6)
  end

  it "returns the input when given a Month, since it's a value object" do
    month = Month.from(Month.new(2014, 6))
    expect(month).to eq Month.new(2014, 6)
  end
end

describe Month, ".now" do
  it "is the current month" do
    Timecop.freeze(Time.zone.local(2014, 1, 1,  0, 0, 0)) do
      expect(Month.now).to eq Month.new(2014, 1)
    end
  end
end

describe Month, "#first_date" do
  it "is the first date of the month" do
    month = Month.new(2014, 2)
    expect(month.first_date).to eq Date.new(2014, 2, 1)
  end
end

describe Month, "#last_date" do
  it "is the last date of the month" do
    month = Month.new(2014, 2)
    expect(month.last_date).to eq Date.new(2014, 2, 28)
  end
end

describe Month, "#last_day" do
  it "is the last day number of the month" do
    month = Month.new(2014, 2)
    expect(month.last_day).to eq 28
  end
end

describe Month, "#date_for_day" do
  it "gives you such a date" do
    date = Month.new(2014, 1).date_for_day(2)
    expect(date).to eq Date.new(2014, 1, 2)
  end
end
 
describe Month, "#previous" do
  it "is the previous month" do
    month = Month.new(2014, 2)
    expect(month.previous).to eq Month.new(2014, 1)
    expect(month.previous.previous).to eq Month.new(2013, 12)
  end
end

describe Month, "#next" do
  it "is the next month" do
    month = Month.new(2013, 11)
    expect(month.next).to eq Month.new(2013, 12)
    expect(month.next.next).to eq Month.new(2014, 1)
  end
end

If you use pred and succ rather than previous and next you'll get range support for free...