private static function isValidTimezone(string $timezone): bool
    {
        // @codeCoverageIgnoreStart
        if (self::$timezones === null) {
            self::$timezones = [];
            foreach (timezone_identifiers_list() as $zone) {
                self::$timezones[$zone] = true;
            }
        }
        // @codeCoverageIgnoreEnd

        return isset(self::$timezones[$timezone]);
    }