docs: def activate_crontab(self):
          """Activate polling function and register first crontab
          """
          self._crontab = []
          if hasattr(self, 'CRONTAB'):
              for crontab_spec in self.CRONTAB:
                  args = cronjob.parse_crontab(crontab_spec)
                  job = cronjob.CronJob()
                  if args['_timer'] == 'datetime':
                      job.set_triggers(args['trigger_format'], args['trigger_time'])
                  if args['_timer'] == 'crontab':
                      job.set_crontab(args['crontab'])
                  if args['action'].startswith('.'):
                      action_name = args['action'][1:]
                      action_ = getattr(self.__class__, action_name)
                  else:
                      action_ = args['action']
                  job.set_action(action_, *args['args'])
                  self._crontab.append(job)
          self.start_poller(30, self.poll_crontab)