/**

 * Generate the display of the time taken column.

 * @param object $attempt the table row being output.

 * @return string HTML content to go inside the td.

 */

public function col_duration($attempt) {

    if ($attempt->timefinish) {

        return format_time($attempt->timefinish - $attempt->timestart);

    } else {

        return '-';

    }

}