public function save_user_extension($userid, $extensionduedate) {
        global $DB;

        // Need submit permission to submit an assignment.
        require_capability('mod/assign:grantextension', $this->context);

        if (!is_enrolled($this->get_course_context(), $userid)) {
            return false;
        }
        if (!has_capability('mod/assign:submit', $this->context, $userid)) {
            return false;
        }

        if ($this->get_instance()->duedate && $extensionduedate) {
            if ($this->get_instance()->duedate > $extensionduedate) {
                return false;
            }
        }
        if ($this->get_instance()->allowsubmissionsfromdate && $extensionduedate) {
            if ($this->get_instance()->allowsubmissionsfromdate > $extensionduedate) {
                return false;
            }
        }

        $flags = $this->get_user_flags($userid, true);
        $flags->extensionduedate = $extensionduedate;

        $result = $this->update_user_flags($flags);

        if ($result) {
            \mod_assign\event\extension_granted::create_from_assign($this, $userid)->trigger();
        }
        return $result;
    }