def _CheckTimeField(self, log):
    valid_at_periods = ('reboot', 'yearly', 'annually', 'monthly',
                        'weekly', 'daily', 'midnight', 'hourly')
    if self.time_field not in valid_at_periods:
      log.LineError(log.MSG_INVALID_AT,
                    'Invalid @ directive "%s"' % self.time_field)
Returns
dataset/ETHPy150Open lyda/chkcrontab/chkcrontab_lib.py/CronLineAt._CheckTimeField--crossover with--dataset/ETHPy150Open muricoca/crab/scikits/crab/models/classes.py/MatrixBooleanPrefDataModel.items_from_user