public function get_locktime() {
        $this->load_grade_item();

        $item_locktime = $this->grade_item->get_locktime();

        if (empty($this->locktime) or ($item_locktime and $item_locktime < $this->locktime)) {
            return $item_locktime;

        } else {
            return $this->locktime;
        }
    }