code: public function startTimer(RunnerServiceContext $context, $timestamp = null)
      {
          if ($context instanceof QtiRunnerServiceContext) {
              /* @var TestSession $session */
              $session = $context->getTestSession();
              if($session->getState() === AssessmentTestSessionState::INTERACTING) {
                  $session->startItemTimer($timestamp);
              }
          } else {
              throw new \common_exception_InvalidArgumentType(
                  'QtiRunnerService',
                  'startTimer',
                  0,
                  'oat\taoQtiTest\models\runner\QtiRunnerServiceContext',
                  $context
              );
          }
          return true;
      }
docs: Starts the timer for the current item in the TestSession
  
  @param RunnerServiceContext $context
  @param float $timestamp allow to start the timer at a specific time, or use current when it's null
  @return bool
  @throws \common_exception_InvalidArgumentType