How to fake Time.now?

I really like the Timecop library. You can do time warps in block form (just like time-warp):

Timecop.travel(6.days.ago) do
  @model = TimeSensitiveMode.new
end
assert @model.times_up!

(Yes, you can nest block-form time travel.)

You can also do declarative time travel:

class MyTest < Test::Unit::TestCase
  def setup
    Timecop.travel(...)
  end
  def teardown
    Timecop.return
  end
end

I have some cucumber helpers for Timecop here. They let you do things like:

Given it is currently January 24, 2008
And I go to the new post page
And I fill in "title" with "An old post"
And I fill in "body" with "..."
And I press "Submit"
And we jump in our Delorean and return to the present
When I go to the home page
I should not see "An old post"