Person#age gives the wrong answer for a person who hasn’t had their birthday yet this year

gistfile1.rb

require 'active_support/time'
require 'active_support/testing/time_helpers'
require 'rspec/expectations'
include ActiveSupport::Testing::TimeHelpers
include RSpec::Matchers

class Person < Struct.new(:birthday)
  def age
    Date.today.year - birthday.year
  end
end

today = Date.new(2014, 4, 30) # => Wed, 30 Apr 2014
travel_to today

my_fiftieth_birthday = 1.week.from_now.to_date # => Wed, 07 May 2014
the_day_i_was_born = my_fiftieth_birthday - 50.years # => Thu, 07 May 1964
me = Person.new(the_day_i_was_born) # => #<struct Person birthday=Thu, 07 May 1964>

expect(me.age).to eq 49
# RSpec::Expectations::ExpectationNotMetError:
# expected: 49
#      got: 50

This code is part of How testability can help.