League\Period\Period::createFromYearInterval PHP Method

    protected static function createFromYearInterval($interval, $year, $index)
    {
        $month = sprintf('%02s', (static::validateRange($index, 1, 12 / $interval) - 1) * $interval + 1);
        $startDate = new DateTimeImmutable(static::validateYear($year) . '-' . $month . '-01');
        return new static($startDate, $startDate->add(new DateInterval('P' . $interval . 'M')));
    }