buildStates example

$date_formats[static::CUSTOM_DATE_FORMAT] = $this->t('Custom');

    $time_diff = $this->getSetting('time_diff');

    $form['time_diff']['#tree'] = TRUE;
    $form['time_diff']['enabled'] = [
      '#type' => 'checkbox',
      '#title' => $this->t("Display as a time difference (e.g. '6 months ago')"),
      '#default_value' => $time_diff['enabled'],
    ];

    $states = $this->buildStates(['time_diff', 'enabled'], ['checked' => TRUE]);

    $form['time_diff']['future_format'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Future format'),
      '#description' => $this->t("Use the <code>@interval</code> placeholder to represent the formatted time difference interval. E.g. <code>@interval hence</code> will be displayed as <em>2 hours 5 minutes hence</em>."),
      '#default_value' => $time_diff['future_format'],
      '#states' => $states,
    ];

    $form['time_diff']['past_format'] = [
      '#type' => 'textfield',