public function getOldestCachefileDate()
    {
        if (!is_dir($this->cacheDir)) {
            return null;
        }

        $finder = new Finder();
        $finder->files()
            ->name('catalogue*.php')
            ->sortByModifiedTime()
            ->in($this->cacheDir);

        foreach ($finder as $file) {
            $date = new \DateTime();
            $date->setTimestamp($file->getMTime());

            return $date;
        }

        return null;
    }