def added=(value)
      set_attribute 'added', case value
      when Time; value.strftime '%Y-%m-%d'
      when String; value
      else
        raise ArgumentError
      end
    end
def added=(value)
      set_attribute 'added', case value
      when Time; value.strftime '%Y-%m-%d'
      when String; value
      else
        raise ArgumentError
      end
    end
Sets addition date.