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oleibman
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This change is extracted from PR #2787 by @MarkBaker. That change mostly deals with array functions, and that part will be superseded by PR #3962. However, this part of 2787 is not included in 3962. Fix #1036 (closed as stale in 2019 and just reopened). Excel spreadsheets can have either of 2 base dates, 1900 or 1904, and the numeric value of any date cells will vary depending on which base date is in use. PhpSpreadsheet has, till now, handled that as a static property of Shared/Date. This does not work well if two spreadsheets with different base dates are open simultaneously. The code is changed to store the base date as a property of the spreadsheet when an Xls/Xlsx spreadsheet is loaded, and use that property when saving an Xls/Xlsx spreadsheet. Any call to `getCalculatedValue` or `getFormattedValue` will temporarily set the Shared/Date value to that of the spreadsheet, and restore it at completion. In order to avoid a BC break, the Xls and Xlsx readers will continue to populate the Shared/Date value as before.