public function getMaxAge() {

		if ($this->headers->hasCacheControlDirective('s-maxage')) {
			return (int) $this->headers->getCacheControlDirective('s-maxage');
		}

		if ($this->headers->hasCacheControlDirective('max-age')) {
			return (int) $this->headers->getCacheControlDirective('max-age');
		}

		if (NULL !== $this->getExpires()) {
			return $this->getExpires()->format('U') - $this->getDate()->format('U');
		}

		return NULL;

	}