public function getLastModified()
    {
        if (!count($this->assets)) {
            return;
        }

        $mtime = 0;
        foreach ($this as $asset) {
            $assetMtime = $asset->getLastModified();
            if ($assetMtime > $mtime) {
                $mtime = $assetMtime;
            }
        }

        return $mtime;
    }