get retro id
        :param home_team_id: home team id
        :param timestamp: game day
        :param game_number: game number
        :return: retro id

def _get_retro_id(cls, home_team_id, timestamp, game_number):
        """
        get retro id
        :param home_team_id: home team id
        :param timestamp: game day
        :param game_number: game number
        :return: retro id
        """
        return '{home_team_id}{year}{month}{day}{game_number}'.format(
            **{
                'home_team_id': home_team_id.upper(),
                'year': timestamp.year,
                'month': timestamp.strftime('%m'),
                'day': timestamp.strftime('%d'),
                'game_number': int(game_number)-1,
            }
        )