public function execute() {
        global $CFG, $DB;
        require_once($CFG->dirroot . '/mod/chat/lib.php');

        chat_update_chat_times();
        chat_delete_old_users();

        $timenow = time();
        $subselect = "SELECT c.keepdays
                        FROM {chat} c
                       WHERE c.id = {chat_messages}.chatid";
        $DB->delete_records_select('chat_messages', "($subselect) > 0 AND timestamp < (? - ($subselect) * ?)",
                [$timenow, DAYSECS]);

        $DB->delete_records_select('chat_messages_current', "timestamp < ?", [$timenow - 8 * HOURSECS]);
    }