public function setLastModified($lastModified)
    {
        if ($lastModified instanceof DateTime) {
            $this->lastModified = $lastModified;
            return;
        } elseif ($lastModified instanceof Model) {
            $this->lastModified = $lastModified->updated_at;
            return;
        }

        $this->lastModified = new DateTime($lastModified);
    }