/**
 * Implements hook_user_login().
 */
function {{ machine_name }}_user_login(UserInterface $account) {
  $config = \Drupal::config('system.date');
  // If the user has a NULL time zone, notify them to set a time zone.
  if (!$account->getTimezone() && $config->get('timezone.user.configurable') && $config->get('timezone.user.warn')) {
    \Drupal::messenger()
      ->addStatus(t('Configure your <a href=":user-edit">account time zone setting</a>.', [
        ':user-edit' => $account->url('edit-form', [
          'query' => \Drupal::destination()
            ->getAsArray(),
          'fragment' => 'edit-timezone',
        ]),
      ]));
  }
}