First the "wrong" way then the "right" way, and how that can get out of hand.

gistfile1.rb

class Person
  def age
    Date.now-birthyear
  end
end

test "That a person has an age" do
  p=Person.new 1974
  travel_to 2014
  assert(p.age==40)
end

gistfile2.rb

class Person
  def age(now=Date.now)
    now-birthyear
  end
end

test "that a person has an age" do
  p=Person.new 1974
  assert(p.age(2014)==40)
end

gistfile3.rb

class Person
  def age(now=Date.now)
    now-birthyear
  end
  
  def can_drink_yet?(now=Date.now)
    age(now)>21
  end
end