参考：https://stackoverflow.com/questions/44304795/how-to-retrieve-date-from-table-cell-using-phpspreadsheet
$birthday = date('Y-m-d H:i:s', \PhpOffice\PhpSpreadsheet\Shared\Date::excelToTimestamp($data['生日']));
$birthday = \PhpOffice\PhpSpreadsheet\Shared\Date::excelToDateTimeObject($value)->format('Y-m-d H:i:s');