private function addDays($extraSpec)
    {
        if (preg_match('/(\d+)D$/', $extraSpec, $matches)) {
            $days = end($matches);

            $this->dateInterval = DateInterval::__set_state(
                array(
                    'y' => $this->dateInterval->y,
                    'm' => $this->dateInterval->m,
                    'd' => $this->dateInterval->d,
                    'h' => $this->dateInterval->h,
                    'i' => $this->dateInterval->i,
                    's' => $this->dateInterval->s,
                    'invert' => $this->dateInterval->invert,
                    'days' => $days
                )
            );
        }
    }