RegionalForm::buildForm

public RegionalForm::buildForm(array $form, FormStateInterface $form_state)

public function buildForm(array $form, FormStateInterface $form_state) {
  $countries = $this->countryManager->getList();
  $system_date = $this->config('system.date');

  // Date settings:
  $zones = system_time_zones();

  $form['locale'] = array(
    '#type' => 'details',
    '#title' => t('Locale'),
    '#open' => TRUE,
  );

  $form['locale']['site_default_country'] = array(
    '#type' => 'select',
    '#title' => t('Default country'),
    '#empty_value' => '',
    '#default_value' => $system_date->get('country.default'),
    '#options' => $countries,
    '#attributes' => array('class' => array('country-detect')),
  );

  $form['locale']['date_first_day'] = array(
    '#type' => 'select',
    '#title' => t('First day of week'),
    '#default_value' => $system_date->get('first_day'),
    '#options' => array(0 => t('Sunday'), 1 => t('Monday'), 2 => t('Tuesday'), 3 => t('Wednesday'), 4 => t('Thursday'), 5 => t('Friday'), 6 => t('Saturday')),
  );

  $form['timezone'] = array(
    '#type' => 'details',
    '#title' => t('Time zones'),
    '#open' => TRUE,
  );

  $form['timezone']['date_default_timezone'] = array(
    '#type' => 'select',
    '#title' => t('Default time zone'),
    '#default_value' => $system_date->get('timezone.default') ? : date_default_timezone_get(),
    '#options' => $zones,
  );

  $configurable_timezones = $system_date->get('timezone.user.configurable');
  $form['timezone']['configurable_timezones'] = array(
    '#type' => 'checkbox',
    '#title' => t('Users may set their own time zone'),
    '#default_value' => $configurable_timezones,
  );

  $form['timezone']['configurable_timezones_wrapper'] = array(
    '#type' => 'container',
    '#states' => array(
      // Hide the user configured timezone settings when users are forced to use
      // the default setting.
      'invisible' => array(
        'input[name="configurable_timezones"]' => array('checked' => FALSE),
      ),
    ),
  );
  $form['timezone']['configurable_timezones_wrapper']['empty_timezone_message'] = array(
    '#type' => 'checkbox',
    '#title' => t('Remind users at login if their time zone is not set'),
    '#default_value' => $system_date->get('timezone.user.warn'),
    '#description' => t('Only applied if users may set their own time zone.')
  );

  $form['timezone']['configurable_timezones_wrapper']['user_default_timezone'] = array(
    '#type' => 'radios',
    '#title' => t('Time zone for new users'),
    '#default_value' => $system_date->get('timezone.user.default'),
    '#options' => array(
      DRUPAL_USER_TIMEZONE_DEFAULT => t('Default time zone'),
      DRUPAL_USER_TIMEZONE_EMPTY => t('Empty time zone'),
      DRUPAL_USER_TIMEZONE_SELECT => t('Users may set their own time zone at registration'),
    ),
    '#description' => t('Only applied if users may set their own time zone.')
  );

  return parent::buildForm($form, $form_state);
}