public function col_timemodified($row) {
        if ($row->timemodified) {
            if ($this->is_downloading()) {
                // Use timestamp format readable for both machines and humans.
                return date_format_string($row->timemodified, '%Y-%m-%d %H:%M:%S %Z');
            } else {
                // Use localised calendar format.
                return userdate($row->timemodified, get_string('strftimedatetime'));
            }
        } else {
            return null;
        }
    }