This is:

- [X ] a bug report
- [ ] a feature request
- [ ] **not** a usage question (ask them on https://stackoverflow.com/questions/tagged/phpspreadsheet or https://gitter.im/PHPOffice/PhpSpreadsheet)

What is the expected behavior?

detect returned string when calling fromString then skip operation
(line 132) $value = DateTimeExcel\DateValue::fromString($value) + DateTimeExcel\TimeValue::fromString($value);

What is the current behavior?

string+string is an issue in PHP 8.x
Fatal error: Uncaught TypeError: Unsupported operand types: string + string in C:\Program Files\EasyPHP-Devserver-17\eds-www\aplitec\includes\modules\phpoffice\phpspreadsheet\src\PhpSpreadsheet\Calculation\TextData\Format.php:132

What are the steps to reproduce?

If this is an issue with reading a specific spreadsheet file, then it may be appropriate to provide a sample file that demonstrates the problem; but please keep it as small as possible, and sanitize any confidential information before uploading.

What features do you think are causing the issue

• Reader
• [ X] Writer
• Styles
• Data Validations
• Formula Calculations
• Charts
• AutoFilter
• Form Elements

Does an issue affect all spreadsheet file formats? If not, which formats are affected?

DATE Cell formatting when date cell is still a keyword and not yet a valid date!

Which versions of PhpSpreadsheet and PHP are affected? PHP >= 8.0

BUG_PHPSpreadsheet_PHP8_DATE.xlsx