DateFormatFormBase::form

public DateFormatFormBase::form(array $form, FormStateInterface $form_state)

public function form(array $form, FormStateInterface $form_state) {
  $form['label'] = array(
    '#type' => 'textfield',
    '#title' => 'Name',
    '#maxlength' => 100,
    '#description' => t('Name of the date format'),
    '#default_value' => $this->entity->label(),
  );

  $form['id'] = array(
    '#type' => 'machine_name',
    '#description' => t('A unique machine-readable name. Can only contain lowercase letters, numbers, and underscores.'),
    '#disabled' => !$this->entity->isNew(),
    '#default_value' => $this->entity->id(),
    '#machine_name' => array(
      'exists' => array($this, 'exists'),
      'replace_pattern' => '([^a-z0-9_]+)|(^custom$)',
      'error' => $this->t('The machine-readable name must be unique, and can only contain lowercase letters, numbers, and underscores. Additionally, it can not be the reserved word "custom".'),
    ),
  );
  $form['date_format_pattern'] = array(
    '#type' => 'textfield',
    '#title' => t('Format string'),
    '#maxlength' => 100,
    '#description' => $this->t('A user-defined date format. See the <a href="http://php.net/manual/function.date.php">PHP manual</a> for available options.'),
    '#required' => TRUE,
    '#attributes' => [
      'data-drupal-date-formatter' => 'source',
    ],
    '#field_suffix' => ' <small class="js-hide" data-drupal-date-formatter="preview">' . $this->t('Displayed as %date_format', ['%date_format' => '']) . '</small>',
  );

  $form['langcode'] = array(
    '#type' => 'language_select',
    '#title' => t('Language'),
    '#languages' => LanguageInterface::STATE_ALL,
    '#default_value' => $this->entity->language()->getId(),
  );
  $form['#attached']['drupalSettings']['dateFormats'] = $this->dateFormatter->getSampleDateFormats();
  $form['#attached']['library'][] = 'system/drupal.system.date';
  return parent::form($form, $form_state);
}