Comparing time in test

Recently I have code to build a hash from object. One of the feature is it accepts string year.ago and turn this into a time object.

So during test, I may write something like this:

filter = Filter.create(operand: 10, modifier: 'year.ago')
test_method(filter)

This produce a hash looks like this:

hash = {"$lt" => 10.year.ago }

To test, I compare it with the real hash:

expect(hash).to eq("$lt": 10.years.ago)

Which was wrong because it get evaluate at the time it call so they have some millisecond diff.

To solve this, I use Timecop

Timecop.freeze(Time.now) do
  # now no matter how many time you call 10.years.ago
  # it yields same result
end