code: def get_annotation_date(self, r_term):
          
          annotation_date_list = list(self.graph.triples((r_term, self.spdx_namespace['annotationDate'], None)))
          if len(annotation_date_list) != 1:
              self.error = True
              msg = 'Annotation must have exactly one annotation date.'
              self.logger.log(msg)
              return
          return six.text_type(annotation_date_list[0][2])
docs: Returns annotation date or None if not found.
          Reports error on failure.
          Note does not check value format.