A way to edit the methods of the actual Time class for testing purposes. Useful in situations where a fake class (e.g., https://gist.github.com/3672210 ) can't be substituted, such as when third-party code that uses Time is involved.

gistfile1.rb

# Redefine Time.now to be one day earlier than the real thing.
# N.B. Using the "alias" keyword does not work correctly -- in the following, it would cause infinite stack recursion.  Cf. http://www.ruby-forum.com/topic/135598

class Time
  class << self
    alias_method :original_now, :now

    def now
      original_now - 86400
    end
  end
end

# try calling Time.now now to see the effect

# Put back the original definition.
class Time
  class << self
    alias_method :now, :original_now
  end
end