private function dateIntervalToIntervalSpec(DateInterval $interval)
    {
        $date = sprintf('%uY%uM%uD', $interval->y, $interval->m, $interval->d);
        $time = sprintf('%uH%uM%uS', $interval->h, $interval->i, $interval->s);

        // build extra spec
        $extra = '';
        if ($interval->invert) {
            $extra .= 'I';
        }

        if ($interval->days) {
            $extra .= $interval->days . 'D';
        }

        if (strlen($extra) > 0) {
            $extra = '-' . $extra;
        }

        return sprintf('P%sT%s%s', $date, $time, $extra);
    }