Basically and integration test

payable_decorator.rb

PayableDecorator < ApplicationDecorator
  decorates :payable

  def due_date_in_words
    if payable.due_date > Date.today
      "#{due_date} (#{h.distance_of_time_in_words(payable.due_date, Date.today)} from now)"
    elsif payable.due_date < Date.today
      "#{due_date} (#{h.distance_of_time_in_words(payable.due_date, Date.today)} ago)"
    else
      "#{due_date} (today)"
    end
  end
end

payable_decorator_spec.rb

require 'spec_helper'

describe PayableDecorator do
  let(:payable) { mock 'Payable' }

  subject       { PayableDecorator.new payable }

  describe '#due_date_in_words' do

    it 'returns the date due and the number of days until it is due when the payable is due in future' do
      Timecop.freeze( Chronic.parse("March 22, 2012")) do
        payable.stub(:due_date).and_return 5.days.from_now.to_date

        subject.due_date_in_words.should == "March 27, 2012 (5 days from now)"
      end
    end

    it 'returns the date due and the number of days since it has been due if the payable is late' do
      Timecop.freeze( Chronic.parse("March 22, 2012")) do
        payable.stub(:due_date).and_return 5.days.ago.to_date

        subject.due_date_in_words.should == "March 17, 2012 (5 days ago)"
      end
    end

    it 'returns the date and the text "today" if the payable is due today' do
      Timecop.freeze( Chronic.parse("March 22, 2012")) do
        payable.stub(:due_date).and_return Date.today

        subject.due_date_in_words.should == "March 22, 2012 (today)"
      end
    end
  end
end