class RegionalForm

class RegionalForm extends ConfigFormBase {
    
    /**
     * The country manager.
     *
     * @var \Drupal\Core\Locale\CountryManagerInterface
     */
    protected $countryManager;
    
    /**
     * Constructs a RegionalForm object.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The factory for configuration objects.
     * @param \Drupal\Core\Locale\CountryManagerInterface $country_manager
     *   The country manager.
     */
    public function __construct(ConfigFactoryInterface $config_factory, CountryManagerInterface $country_manager) {
        parent::__construct($config_factory);
        $this->countryManager = $country_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('config.factory'), $container->get('country_manager'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'system_regional_settings';
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getEditableConfigNames() {
        return [
            'system.date',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $countries = $this->countryManager
            ->getList();
        $system_date = $this->config('system.date');
        // Date settings:
        $zones = system_time_zones(NULL, TRUE);
        $form['locale'] = [
            '#type' => 'details',
            '#title' => $this->t('Locale'),
            '#open' => TRUE,
        ];
        $form['locale']['site_default_country'] = [
            '#type' => 'select',
            '#title' => $this->t('Default country'),
            '#empty_value' => '',
            '#default_value' => $system_date->get('country.default'),
            '#options' => $countries,
            '#attributes' => [
                'class' => [
                    'country-detect',
                ],
            ],
        ];
        $form['locale']['date_first_day'] = [
            '#type' => 'select',
            '#title' => $this->t('First day of week'),
            '#default_value' => $system_date->get('first_day'),
            '#options' => [
                0 => $this->t('Sunday'),
                1 => $this->t('Monday'),
                2 => $this->t('Tuesday'),
                3 => $this->t('Wednesday'),
                4 => $this->t('Thursday'),
                5 => $this->t('Friday'),
                6 => $this->t('Saturday'),
            ],
        ];
        $form['timezone'] = [
            '#type' => 'details',
            '#title' => $this->t('Time zones'),
            '#open' => TRUE,
        ];
        $form['timezone']['date_default_timezone'] = [
            '#type' => 'select',
            '#title' => $this->t('Default time zone'),
            '#default_value' => $system_date->get('timezone.default') ?: date_default_timezone_get(),
            '#options' => $zones,
        ];
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->config('system.date')
            ->set('country.default', $form_state->getValue('site_default_country'))
            ->set('first_day', $form_state->getValue('date_first_day'))
            ->set('timezone.default', $form_state->getValue('date_default_timezone'))
            ->save();
        parent::submitForm($form, $form_state);
    }

}