public function execute() {
        global $CFG, $DB;

        if (!empty($CFG->gradehistorylifetime)) {
            $now = time();
            $histlifetime = $now - ($CFG->gradehistorylifetime * DAYSECS);
            $tables = [
                'grade_outcomes_history',
                'grade_categories_history',
                'grade_items_history',
                'grade_grades_history',
                'scale_history'
            ];
            foreach ($tables as $table) {
                if ($DB->delete_records_select($table, "timemodified < ?", [$histlifetime])) {
                    mtrace("    Deleted old grade history records from '$table'");
                }
            }
        }
    }