A bad idea

created_at.rb

module CreatedAt
  module ClassMixin
    def new(*)
      obj = super
      obj.instance_variable_set(:@_created_at, Time.now)

      obj
    end
  end

  module ObjectMixin
    def created_at
      @_created_at
    end
  end
end

class Class
  prepend CreatedAt::ClassMixin
end

class Object
  include CreatedAt::ObjectMixin
end