Symfony\Component\HttpFoundation\Response::getTtl PHP Method

    public function getTtl()
    {
        if (null !== $maxAge = $this->getMaxAge()) {
            return $maxAge - $this->getAge();
        }
    }

 /**
  * {@inheritdoc}
  */
 public function updateMetadata(Response $response, BlockContextInterface $blockContext = null)
 {
     if ($this->currentTtl === null) {
         $this->currentTtl = $response->getTtl();
     }
     if ($response->isCacheable() !== null && $response->getTtl() < $this->currentTtl) {
         $this->currentTtl = $response->getTtl();
     }
 }