public function add(DateInterval $interval)
	{
		$years   = $this->y + $interval->y;
		$months  = $this->m + $interval->m;
		$days    = $this->d + $interval->d;
		$hours   = $this->h + $interval->h;
		$minutes = $this->i + $interval->i;
		$seconds = $this->s + $interval->s;

		$spec = sprintf('P%sY%sM%sDT%sH%sM%sS', $years, $months, $days, $hours, $minutes, $seconds);

		return new DateInterval($spec);
	}