gistfile1.rb

class Time
  
  cattr_accessor :frozen_to
  
  class << self 
    
    def now_with_freezing
      if @@frozen_to
        @@frozen_to
      else
        now_without_freezing
      end
    end
    
    alias_method_chain :now, :freezing
  
  end
  
end

def freeze_time(time = Time.now)
  Time.frozen_to = time
  yield
ensure
  Time.frozen_to = nil
end

# == watch the magic ==

freeze_time do
  puts Time.now
  sleep 5
  puts Time.now
end