DateTime 4 years difference because Date::setExcelCalendar

[ybelenko]

This is:

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

I've spent almost 5 hours to investigate weird bug. I've made a small parser application and covered it with unit tests. I didn't understand why my assert of DateTimes fails when I add new test to completely unrelated class. As it turned out PHPUnit runs all tests simultaneously, so if one test contains reading of file in .numbers format and second test handles .xlsx then the PhpOffice\PhpSpreadsheet\Shared\Date::$excelCalendar value will be self::CALENDAR_WINDOWS_1900 for both tests which is wrong.

What is the expected behavior?

I don't know how to fix this issue with current class architecture, it's obviously you cannot switch from static method to internal one.

What is the current behavior?

When I load Mac file and then Win file I expect to calculate DateTime from Mac file correctly.

What are the steps to reproduce?

1. Load spreadsheet A file created on Windows system.
2. Load spreadsheet B file created on Mac system.
3. Get value from datetime cell of the file A and convert it to DateTime with Date::excelToDateTimeObject method. Resulted DateTime instance will be different from original file(4 years different to be more specific).

Which versions of PhpSpreadsheet and PHP are affected?

• PHP 7.4.6 (cli) (built: May 28 2020 23:40:38) ( NTS )
• "phpoffice/phpspreadsheet": "^1.14"

[stale]

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