Like Timecop, but simpler.

time.rb

class Time
  class << self
    attr_accessor :mock_time

    def now_with_mock_time
      @mock_time || now_without_mock_time
    end

    alias_method_chain :now, :mock_time

    # Freeze Time.now, DateTime.now and Date.today to the given Time
    # This method operates only in the context of a provided block.
    #
    # Example Usage:
    #
    #   Time.be(:utc, 2007, 5, 2) do
    #     puts Time.now
    #   end
    #   # => 2007-05-02 00:00:00 UTC
    #
    #   Time.be(:local, 2007, 5, 2, 17, 31, 2) do
    #     puts Time.now
    #   end
    #   # => 2007-05-02 17:31:02 +1000
    #
    # It works with DateTime.now too:
    #
    #   Time.be(Time.utc(2012, 1, 1)) do
    #     puts DateTime.now
    #   end
    #   # => 2012-01-01T00:00:00+00:00
    #
    # And also with Date.today:
    #
    #   Time.be(:utc, 2000, 2, 14) do
    #     puts Date.today
    #   end
    #   # => 2000-02-14
    #
    # Blocks can be nested together:
    #
    #   Time.be(:utc, 2007, 5, 2) do
    #     Time.be(:local, 2007, 5, 2, 17, 31, 2) do
    #       puts Time.now
    #     end
    #     puts Time.now
    #   end
    #   # => 2007-05-02 17:31:02 +1000
    #   # => 2007-05-02 00:00:00 UTC
    #
    # And lastly, you can take advantage of ActiveSupport's Numeric helpers:
    #
    #   Time.be(:utc, 2000, 1, 1) do
    #     Time.be(4.days.from_now.utc) do
    #       puts Time.now
    #       Time.be(3.minutes.ago.utc) do
    #         puts Time.now
    #       end
    #     end
    #   end
    #   # => 2000-01-05 00:00:00 UTC
    #   # => 2000-01-04 23:57:00 UTC
    #
    def be(a_time_or_type, *args)
      raise ArgumentError, "Time.be requires a block, but none was given" unless block_given?

      a_time = if a_time_or_type.kind_of?(Time)
        a_time_or_type
      else
        unless [:utc, :local].include?(a_time_or_type)
          raise ArgumentError, "First argument must either by a Time, :utc or :local"
        end
        Time.send(a_time_or_type, *args)
      end

      original_mock_time = @mock_time
      @mock_time = a_time
      yield
    ensure
      @mock_time = original_mock_time
    end

    # Simply freeze the time at whatever time it is now
    def freeze(&block)
      be(Time.now, &block)
    end
  end
end

class DateTime
  class << self
    def now(*)
      Time.now.to_datetime
    end
  end
end

class Date
  class << self
    def today(*)
      Time.now.to_date
    end
  end
end