system_form_user_register_form_alter

system_form_user_register_form_alter(&$form, FormStateInterface $form_state)

function system_form_user_register_form_alter(&$form, FormStateInterface $form_state) {
  $config = \Drupal::config('system.date');
  if ($config->get('timezone.user.configurable') && $config->get('timezone.user.default') == DRUPAL_USER_TIMEZONE_SELECT) {
    system_user_timezone($form, $form_state);
  }
}