Class to represent a short date (MM/DD), without the year component

short_date.rb

require 'date'

class ShortDate
  attr_reader :month, :day
  include Comparable

  def initialize(string)
    date = Date.strptime(string, '%m/%d')
    @month, @day = date.month, date.day
  rescue StandardError
  end

  def ==(other_day)
    month == other_day.month && day == other_day.day
  end

  def <=>(other_day)
    if month == other_day.month
      day <=> other_day.day
    else
      month <=> other_day.month
    end
  end

  def to_s
    '%02u/%02u' % [month, day] if month && day
  end

  def valid?
    @month && @day
  end

end

short_date_spec.rb

require 'spec_helper'
require './lib/short_date'

RSpec.describe ShortDate do
  it 'has day and month attributes' do
    date = ShortDate.new("1/2")
    expect(date.month).to eq 1
    expect(date.day).to eq 2
  end

  it 'can be converted to a string' do
    expect(ShortDate.new("10/11").to_s).to eq '10/11'
    # invalid dates
    expect(ShortDate.new("20/20").to_s).to be_nil
  end

  it 'is comparabable' do
    # when date1.month < date2.month
    expect(ShortDate.new("1/2")).to be <= ShortDate.new("2/1")
    expect(ShortDate.new("1/2")).to be <= ShortDate.new("2/2")
    expect(ShortDate.new("1/2")).to be <= ShortDate.new("2/3")

    # when date1.month == date2.month
    expect(ShortDate.new("2/2")).to be >= ShortDate.new("2/1")
    expect(ShortDate.new("2/2")).to be == ShortDate.new("2/2")
    expect(ShortDate.new("2/2")).to be <= ShortDate.new("2/3")

    # when date1.month > date2.month
    expect(ShortDate.new("3/2")).to be >= ShortDate.new("2/1")
    expect(ShortDate.new("3/2")).to be >= ShortDate.new("2/2")
    expect(ShortDate.new("3/2")).to be >= ShortDate.new("2/3")

    # with leading zeroes
    expect(ShortDate.new('04/06')).to be == ShortDate.new("4/6")
    # '4/6/2000' still matches '%m/%d' ...
    expect(ShortDate.new('4/6/2000')).to be == ShortDate.new("4/6")
  end

  it 'is validable' do
    expect(ShortDate.new("1/1")).to be_valid

    # invalid dates
    expect(ShortDate.new("20/1")).to_not be_valid
    expect(ShortDate.new("1/32")).to_not be_valid
    expect(ShortDate.new("2/30")).to_not be_valid

    # any other format will fail
    expect(ShortDate.new('2000/4/6')).to_not be_valid
    expect(ShortDate.new('Apr 6')).to_not be_valid
  end
end

Example with Rails:

class RecurringEvent < ActiveRecord::Base
  composed_of :date, class_name: "ShortDate"
end

A simpler implementation, but without day/month attributes

require 'date'

class ShortDate
  attr_reader :date
  include Comparable

  def initialize(string)
    temp = Date.strptime(string, '%m/%d')
    @date = '%02u/%02u' % [temp.month, temp.day]
  rescue StandardError
  end

  def ==(other_day)
    date == other_day.date
  end

  def <=>(other_day)
    date <=> other_day.date
  end

  def to_s
    date
  end

  def valid?
    !date.nil?
  end

end