docs: def _format_regular_value(self, str_in):
          # type: (Text) -> Text
          """ we overwrite default behaviour as we want to hash the numbers
          only, no fillers like '-', or '/'
  
          :param str str_in: date string
          :return: str date string with format DateSpec.OUTPUT_FORMAT
          """
          try:
              dt = datetime.strptime(str_in, self.format)
              return strftime(dt, DateSpec.OUTPUT_FORMAT)
          except ValueError as e:
              msg = "Unable to format date value '{}'. Reason: {}".format(str_in,
                                                                          e)
              e_new = InvalidEntryError(msg)
              e_new.field_spec = self
              raise_from(e_new, e)