    protected function convertDate($startDate, $format)
    {
        $date = "";

        if ($startDate instanceof \DateTime) {
            switch ($format) {
                case self::FORMAT_YYMMDD:
                    $date = $startDate->format('ymd');
                    break;
                case self::FORMAT_MMDD:
                    $date = $startDate->format('md');
                    break;
                case self::FORMAT_YYDDD:
                    $date = $startDate->format('yz');
                    if (strlen($date) === 4) {
                        $date = substr($date, 0, 2)."0".substr($date, 2);
                    }
                    break;
                default:
                    throw new \RuntimeException("Installments Format '".$format."' is not implemented!");
            }
        }

        return $date;
    }