How to fake Time.now?

Question

What\'s the best way to set Time.now for the purpose of testing time-sensitive methods in a unit test?

Answer 1

The recently-released Test::Redef makes this and other fakery easy, even without restructuring the code in a dependency-injection style (especially helpful if you're using other peoples' code.)

fake_time = Time.at(12345) # ~3:30pm UTC Jan 1 1970
Test::Redef.rd 'Time.now' => proc { fake_time } do
   assert_equal 12345, Time.now.to_i
end

However, be careful of other ways to obtain time that this will not fake out (Date.new, a compiled extension that makes its own system call, interfaces to things like external database servers which know current timestamps, etc.) It sounds like the Timecop library above might overcome these limitations.

Other great uses include testing things like "what happens when I'm trying to use this friendly http client but it decides to raise this an exception instead of returning me a string?" without actually setting up the network conditions which lead to that exception (which may be tricky). It also lets you check the arguments to redef'd functions.