//$sheet->setCellValue($currentLetter . $row, $temporary_start->format('G:i:00'));
//$sheet->setCellValue($currentLetter . $row, \PhpOffice\PhpSpreadsheet\Shared\Date::PHPToExcel($temporary_start->getTimestamp()));

$sheet->getStyle($currentLetter . $row)
      ->getNumberFormat()
      ->setFormatCode(\PhpOffice\PhpSpreadsheet\Style\NumberFormat::FORMAT_DATE_TIME4);
    //->setFormatCode('gg:mm');