This is a post in my series on things I've remarked on in code review.

Do you see a problem with this test?

describe EventFinder, ".find_all_on_date" do
  it "includes events that happen on that date" do
    event = create(Event, happens_on: Time.zone.today)
    found_events = EventFinder.find_all_on_date(Time.zone.today)
    expect(found_events).to include(event)
  end
end

What happens if this test runs right around midnight, or around a daylight saving transition?

The event might be created just as Monday ends, and EventFinder.find_all_on_date may run just as Tuesday starts.

So we'd create a Monday event but search for Tuesday events, and the test will fail.

In this case the fix is trivial:

describe EventFinder, ".find_all_on_date" do
  it "includes events that happen on that date" do
    today = Time.zone.today
    event = create(Event, happens_on: today)
    found_events = EventFinder.find_all_on_date(today)
    expect(found_events).to include(event)
  end
end

In more complex cases, you may need to call a time cop and ask them to freeze time for the duration of your test.

Admittedly, this isn't a major issue. I see these types of tests failures a few times a year – it doesn't happen often if you don't often run your tests around midnight.

But it's also easy to avoid once you're aware of it, so I recommend writing tests not to be midnight fragile from now on.