Time period that is determined from the arguments --date and --last. It's a 2-tuple with
        (<start datetime>, <end_datetime>) items. An item is `None` if there isn't a limit.

def time_period(self):
        """
        Time period that is determined from the arguments --date and --last. It's a 2-tuple with
        (<start datetime>, <end_datetime>) items. An item is `None` if there isn't a limit.
        """
        if self.args.time_period is None:
            if self.args.files or is_pipe(STDIN_FILENO) or is_redirected(STDIN_FILENO):
                time_period = (None, None)
            else:
                diff = 86400  # 24h = 86400 seconds
                time_period = get_datetime_interval(int(time.time()), diff, 3600)
        else:
            time_period = self.args.time_period

        logger.debug('time period to be processed: %r', time_period)
        return time_period