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
  end
end

time_spec.rb

require 'spec_helper'

describe "Time.be" do
  it "sets the current time for the scope of a block" do
    Time.be(Time.utc(2009, 3, 9)) { Time.now.should == Time.utc(2009, 3, 9) }
  end
  
  it "supports nesting blocks" do
    Time.be(Time.utc(2009, 3, 9)) do
      Time.be(Time.utc(2007, 1, 1)) { Time.now.should == Time.utc(2007, 1, 1) }
    end
  end
  
  it "restores the time after the block has executed" do
    Time.be(Time.utc(2009, 3, 9)) do
      Time.be(Time.utc(2007, 1, 1)) {  }
      Time.now.should == Time.utc(2009, 3, 9)
    end
  end
  
  it "supports passing time values as arguments" do
    Time.be(:local, 2009, 3, 9, 17, 37, 59) do
      Time.now.should == Time.local(2009, 3, 9, 17, 37, 59)
    end
  end
  
  it "allows travelling through relative time" do
    Time.be(:utc, 2009, 3, 9) do
      Time.be(2.hours.from_now) do
        Time.now.should == Time.utc(2009, 3, 9, 2, 0, 0)
        Time.be(4.days.ago) do
          Time.now.should == Time.utc(2009, 3, 5, 2, 0, 0)
        end
      end
    end
  end
  
  it "affects DateTime" do
    Time.be(:utc, 2009, 3, 9) do
      date_time = DateTime.now
      date_time.year.should == 2009
      date_time.month.should == 3
      date_time.day.should == 9
      date_time.hour.should == 0
      date_time.minute.should == 0
    end
  end
  
  it "affects Date" do
    Time.be(:utc, 2009, 3, 9) do
      date = Date.today
      date.year.should == 2009
      date.month.should == 3
      date.day.should == 9
    end
  end
end

Comment no longer makes sense. Should have really used a new gist. Meh.