format_date_range.rb

require "active_support/all"

def format_date_range(from, to = false)
  to = from if to.blank?
  return '' if from.nil? && to.nil?
  from, to = [to, from].sort

  if from == to
    month_day_year(from)
  elsif from.year == to.year
    format_date_range_in_the_same_year(from, to)
  elsif from.beginning_of_month == from && to.end_of_month == to
    "#{month_year(from)} – #{month_year(to)}"
  else
    "#{month_day_year(from)} – #{month_day_year(to)}"
  end.squish
end

def format_date_range_in_the_same_year(from, to)
  if from.month == to.month && from.beginning_of_month == from && to.end_of_month == to
    month_year(from)
  elsif from.beginning_of_month == from && to.end_of_month == to
    "#{from.strftime('%B')}–#{month_year(to)}"
  elsif from.month == to.month
    "#{from.strftime('%B %e')}–#{to.strftime('%e, %Y')}"
  else
    "#{from.strftime('%B %e')} – #{month_day_year(to)}"
  end
end

def month_year(date)
  date.strftime('%B %Y')
end

def month_day_year(date)
  date.strftime('%B %e, %Y')
end

format_date_range_test.rb

require "./format_date_range"
require "date"

require "minitest/autorun"

describe "#format_date_range" do
  describe "date ranges, including year threshold and full month" do
    it "formats dates in the same month" do
      assert_equal "April 2–17, 2014",
        format_date_range(Date.new(2014,4,2),Date.new(2014,4,17))
    end

    it "formats dates in different months and years." do
      assert_equal "December 13, 2013 – January 4, 2014",
        format_date_range(Date.new(2013,12,13),Date.new(2014,1,4))
    end

    it "formats dates at the beginning and end of different months" do
      assert_equal "May–June 2014",
        format_date_range(Date.new(2014,5,1),Date.new(2014,6,30))
    end

    it "formats dates at the beginning and end of different months in different years" do
      assert_equal "October 2013 – June 2014",
        format_date_range(Date.new(2013,10,1),Date.new(2014,6,30))
    end

    it "formats the first and last of a month to the month itself" do
      assert_equal "April 2014",
        format_date_range(Date.new(2014,4,1),Date.new(2014,4,30))
    end
  end

  describe "dates in reverse order" do
    it "formats dates in the same month" do
      assert_equal "April 2–17, 2014",
        format_date_range(Date.new(2014,4,17),Date.new(2014,4,2))
    end

    it "formats dates in different months and years" do
      assert_equal "December 13, 2013 – January 4, 2014",
        format_date_range(Date.new(2014,1,4),Date.new(2013,12,13))
    end

    it "formats the last and first date of the month to be the month" do
      assert_equal "April 2014",
        format_date_range(Date.new(2014,4,30),Date.new(2014,4,1))
    end
  end

  describe "same day" do
    it "responds with that date" do
      assert_equal "April 4, 2014",
        format_date_range(Date.new(2014,4,4),Date.new(2014,4,4))
    end
  end

  describe "alternate arguments" do
    it "takes nil as its second argument" do
      assert_equal format_date_range(Date.new(2014,4,4), nil), "April 4, 2014"
    end

    it "takes one argument" do
      assert_equal format_date_range(Date.new(2014,4,4)), "April 4, 2014"
    end

    it "takes nil as its first argument" do
      assert_raises(ArgumentError) { format_date_range(nil, Date.new(2014,4,4)) }
    end

    it "takes nil as both arguments" do
      assert_equal format_date_range(nil, nil), ""
    end
  end
end