How to override Date.today when running rails tests?

Question

I have some tests that were failing on specific dates, because someone wrote them to use Date.today. I want to reproduce the failures on previous select dates.

Is there a way to run rake test with an ENV variable that will override the system clock? So that calls to Date.today, Time.now, and 1.day.ago and 1.day.from_now will use the date I set?

Like, for example:

> DATE_TODAY='2017-01-04' rake test

Answer 1

For testing you can use timecop gem. It offers you two useful methods Timecop.freeze and Timecop.travel.

For example, you can use freeze to statically set time in before hooks:

describe 'your tests' do
  before do
    Timecop.freeze(Time.new(2017, 1, 4))
  end

  after do
    Timecop.return
  end

  it 'should do something' do
    sleep(10)
    Time.now # => 2017-01-04 00:00:00
  end
end

Or in a block:

Timecop.freeze(Time.now - 3.months) do
  assert product.expired?
end

While with the travel method, you change the starting moment, but time is still passing by.

Timecop.travel(Time.new(2017, 1, 4))
sleep(10)
Time.now # => 2017-01-04 00:00:10

Answer 2

As of Rails 4.1 you can do

travel_to Time.new(2004, 11, 24, 01, 04, 44)

The full API docs are here: http://api.rubyonrails.org/classes/ActiveSupport/Testing/TimeHelpers.html