In what format is ActiveJob :at key

I am trying to understand in what format :at for ActiveJob is, so I can use assert_enqueued_with with :at parameter.

Currently I've got this test which passes:

tomorrow = 1.day.from_now.to_date
assert_enqueued_with(job: ActionMailer::DeliveryJob) do
  onboarding.start_onboarding
  job_date = ActiveJob::Base.queue_adapter.enqueued_jobs[0][:at]
  assert_equal tomorrow, Time.at(job_date).to_date
end

But then I've realised that assert_enqueued_with has :at parameter and I would prefer to use it instead of last two lines in the block. Problem is, tomorrow variable seems to be in a different format than :at, which makes this fail:

tomorrow = 1.day.from_now.to_date
assert_enqueued_with(job: ActionMailer::DeliveryJob, at: tomorrow) do
  onboarding.start_onboarding
end

with this error:

Minitest::Assertion: No enqueued job found with {:job=>ActionMailer::DeliveryJob, :at=>Wed, 31 May 2017 13:12:05 UTC +00:00}

As you can see, tomorrow is a date, say Wed, 31 May 2017, while job_date is some kind of a large number, say 1496236366.183789. Hence I am trying to convert tomorrow into the same format.

Would really appreciate some help with this. Perhaps I am using :at wrongly or you know what format :at in and help me convert tomorrow.

start_onboarding is just a AASM event, which calls this method to schedule email delivery (a bit modified for privacy reasons):

def notify(id)
  person = User.find(id) 
  PersonMailer.onboarding_reminder(person).
    deliver_later(wait_until: 1.day.from_now)
end