Refactoring Method after watching a Ben Orenstein OO talk.

After.rb

  def calc_earnings_change_history(number_of_years)
    earnings_history = Array.new
   
    number_of_months(number_of_years).each_with_index do |month_as_integer, index|      
      this_month = month_as_integer.months.ago.end_of_month
      next_month = this_month.next_month
      last_month = this_month.prev_month

      percent_change = index == 0 ? 0.0 : percent_change_between_months(share_price_on(last_month), share_price_on(this_month))
      
      earnings_history << [chart_time_for(this_month), percent_change]
    end
    earnings_history
  end

  def chart_time_for(time)
    time.to_i * 1000
  end

  def percent_change_between_months(last_month, this_month)
    last_month.percent_change(this_month).to_f / 100
  end

  def number_of_months(number_of_years)
    months = number_of_years * 12
    0..months
  end

Before.rb

  def calc_earnings_change_history(number_of_years)
    response = Array.new
    months = number_of_years * 12
    number_of_months = 0..months
    number_of_months.to_a.reverse.each_with_index do |month_offset, index|      
      date = month_offset.months.ago.end_of_month
      this_month = date
      next_month = date.next_month
      this_month_share_price = share_price_on(this_month).to_f
      if index == 0
        percent_change = 0.0
      else
        last_month = this_month.prev_month
        last_month_share_price = share_price_on(last_month).to_f
        percent_change = last_month_share_price.percent_change(this_month_share_price) / 100
      end
      response << [this_month.to_i * 1000, percent_change]
    end
    response
  end