year_diff.rb

require 'test/unit'
require 'date'

def year_diff(a, b)
  month_diff = (a.year * 12 + a.month) - (b.year * 12 + b.month)
  month_diff -= 1 if a.month == b.month && a.day < b.day

  month_diff / 12
end

class TestYearDiff < Test::Unit::TestCase
  def test_year_diff
    assert_equal 1,
      year_diff(Date.parse('2013-01-01'), Date.parse('2012-01-01'))

    assert_equal 1,
      year_diff(Date.parse('2013-01-01'), Date.parse('2011-12-31'))

    assert_equal 0,
      year_diff(Date.parse('2013-01-01'), Date.parse('2012-01-02'))

    assert_equal 1,
      year_diff(Date.parse('2012-01-01'), Date.parse('2011-01-01'))

    assert_equal 1,
      year_diff(Date.parse('2012-01-01'), Date.parse('2010-12-31'))

    assert_equal 0,
      year_diff(Date.parse('2012-01-01'), Date.parse('2011-01-02'))
  end
end