public static function compare(AbstractCalendarDate $d1, AbstractCalendarDate $d2): int
    {
        if ($d1->maximum_julian_day < $d2->minimum_julian_day) {
            return -1;
        }

        if ($d2->maximum_julian_day < $d1->minimum_julian_day) {
            return 1;
        }

        return 0;
    }