person_spec.rb

require 'rspec'
require 'date'
require 'ostruct'

class Person < OpenStruct
	def age
		now.year - birthday.year
	end

	def now
		@now ||= DateTime.now
	end
end

describe Person do
	let(:ju) { Person.new(birthday: Date.new(1986)) }

	context "test like it's 2014" do
		describe '#age' do
			it 'returns its age' do
				ju.now = Date.new(2014)
				expect(ju.age).to eq(28)
			end
		end
	end
end