snippet.rb

require 'date'

module DateDiff

  module_function
  def difference_in_months(d1, d2)
    DateDifference.new(d1,d2).months
  end
  
  class DateDifference
    def initialize(d1, d2)
      @d1, @d2 = [d1, d2].map {|a| coerce_arg(a)}

      @positive = @d1 > @d2
      @d1, @d2  = @d2, @d1 unless positive?
    end
    
    def months
      month_difference = if @d1.month >= @d2.month
        month_difference = @d1.month - @d2.month
      else
        month_difference = 12 - @d2.month + @d1.month
      end

      months = 12 * years + month_difference
      positive? ? months : months * -1
    end

    def years
      if @d1.month >= @d2.month
        @d1.year - @d2.year
      else
        @d1.year - @d2.year - 1
      end
    end

    def positive?
      !!@positive
    end

    protected    
    def coerce_arg(arg)
      case arg
      when String
        Date.parse(arg)
      when Date
        arg
      else
        raise ArgumentError, 'Expecting some sort of date data'
      end
    end
  end
end

require 'test/unit'

module Test
  class DateTests < Test::Unit::TestCase
    def setup
      @els = Date.parse('Jul 2005')
      @msd = Date.parse('Apr 2002')
    end
    
    include DateDiff
    
    def test_one_month
      assert_equal 1, difference_in_months('Jul 2005', 'Jun 2005')
    end
    
    def test_neg_on_month
      assert_equal -1, difference_in_months('Jul 2005', 'Aug 2005')
    end
    
    def test_year_fakeout
      assert_equal 2, difference_in_months('Jan 2009', 'Nov 2008')
    end
    
    def test_example
      assert_equal 39, difference_in_months(@els, @msd)
      assert_equal 308, difference_in_months(@els, @msd)*8-4
    end
  end
end