gistfile1.rb

#!/usr/bin/env ruby
# coding: utf-8

require "date"

def default_time(now=DateTime.now)
  @default_time ||= now
end

module ShowDefaultTime
  def show_default_time(t=default_time)
    t
  end
end

class Test
  extend ShowDefaultTime

  def self.test
    show_default_time
  end
end

default_time DateTime.new 2013, 12, 9, 5, 0, 0

#
# 結果が同じになる想定だったが異なる
# (例は 2013/12/18 11:30:10 に実行した結果)
# 
p default_time # => <DateTime: 2013-12-09T05:00:00+00:00 ((2456636j,18000s,0n),+0s,2299161j)>
p Test.test    # => <DateTime: 2013-12-18T11:30:10+09:00 ((2456645j,9010s,735935000n),+32400s,2299161j)>