Can't find views default_argument 'date'

I'm marking this a support request because I can't believe its a bug which hasn't been reported. Nonetheless:
In Drupal\views\Plugin\views\argument\Date:

  public function defaultArgumentForm(&$form, FormStateInterface $form_state) {
    $form['default_argument_type']['#options'] += array('date' => $this->t('Current date ....'));

Seems to imply that there is a default_argument plugin called 'date' But when I select that plugin I get an error and I can't find any such plugin in drupal core.