code: protected function endItemTimer($timestamp = null)
      {
          if($this->getRequestParameter('itemDuration')){
              $serviceContext    = $this->getServiceContext();
              $itemDuration      = $this->getRequestParameter('itemDuration');
              return $this->getRunnerService()->endTimer($serviceContext, $itemDuration, $timestamp);
          }
          return false;
      }
docs: End the item timer to QtiTimeLine
  
  @param null $timestamp The start of action, optional
  @return bool