protected function dueFactor()
    {
        $date = \DateTime::createFromFormat('Y-m-d', $this->title->due);
        $epoch = new \DateTime('1997-10-07');
        if ($date && $date > $epoch) {
            $diff = substr($date->diff($epoch)->format('%a'), -4);
            return str_pad($diff, 4, '0', STR_PAD_LEFT);
        }
        return '0000';
    }