code: def set_created_date(self, doc, created):
          
          if not self.created_date_set:
              self.created_date_set = True
              date = utils.datetime_from_iso_format(created)
              if date is not None:
                  doc.creation_info.created = date
                  return True
              else:
                  raise SPDXValueError('CreationInfo::Date')
          else:
              raise CardinalityError('CreationInfo::Created')
docs: Sets created date, Raises CardinalityError if
          created date already set.
          Raises SPDXValueError if created is not a date.