private function getActualLastModifiedTime()
    {
        if (empty($this->objectList)) return;

        foreach ($this->objectList as $object) {
            $modifiedTime = $object->getLastModifiedTimestamp();

            if (!empty($modifiedTime) && $modifiedTime > $this->lastModifiedTime) {
                $this->lastModifiedTime = $modifiedTime;
            }
        }
    }