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);
  }

}