class ChangedTestItem

class ChangedTestItem extends ChangedItem {
  
  /**
   * {@inheritdoc}
   */
  public function preSave() : void {
    parent::preSave();
    if ($this->value == \Drupal::time()->getRequestTime()) {
      // During a test the request time is immutable. To allow tests of the
      // algorithm of
      // Drupal\Core\Field\Plugin\Field\FieldType\ChangedItem::preSave() we need
      // to set a real time value here. For the stability of the test, set the
      // time of the original language to the current time plus just over one
      // second to simulate two different request times.
      // @todo mock the time service in https://www.drupal.org/node/2908210.
      if ($this->getEntity()
        ->language()
        ->isDefault()) {
        // Wait 1.1 seconds because time_sleep_until() is not reliable.
        time_sleep_until(time() + 1.1);
      }
      $this->value = time();
    }
  }

}