private static function nbrConversionFormat($xVal, $places)
    {
        if (!is_null($places)) {
            if (strlen($xVal) <= $places) {
                return substr(str_pad($xVal, $places, '0', STR_PAD_LEFT), -10);
            } else {
                return PHPExcel_Calculation_Functions::NaN();
            }
        }

        return substr($xVal, -10);
    }