public static function createPeriodKey($period): string
    {
        if ($period->hasFixedDateTimes()) {
            return "{$period->getStartDateTimeString()}|{$period->getEndDateTimeString()}";
        }

        list($subType, $subValueType) = explode('_', strtolower($period->getSubType()));

        return "{$subType}{$period->getSubValue()}{$subValueType}|";
    }