public function readUNBDateTimeOfPreparation()
    {
        // separate date (YYMMDD) and time (HHMM)
        $date = $this->readEdiDataValue('UNB', 4, 0);
        if (!empty($date)) {
            $time = $this->readEdiDataValue('UNB', 4, 1);
            $datetime = \preg_replace('#(\d\d)(\d\d)(\d\d)(\d\d)(\d\d)#', '20$1-$2-$3 $4:$5:00', $date . $time);

            return $datetime;
        }

        // common YYYYMMDDHHMM
        $datetime = $this->readEdiDataValue('UNB', 4);
        $datetime = \preg_replace('#(\d\d\d\d)(\d\d)(\d\d)(\d\d)(\d\d)#', '$1-$2-$3 $4:$5:00', $datetime);

        return $datetime;
    }