# File lib/vpim/vcard.rb, line 1204
      def birthday=(birthday)
        if !birthday.respond_to? :month
          raise ArgumentError, 'birthday must be a date or time object.'
        end
        delete_if { |l| l.name == 'BDAY' }
        @card << Vpim::DirectoryInfo::Field.create( 'BDAY', birthday );
      end