Modify the general settings ExcelCalendar to be simply a default, Create a new ExcelCalendar property for Spreadsheets, and modify the Xlsx and Xls Readers/Writers to maintain 1900 or 1904 calendar against the spreadsheet

src/PhpSpreadsheet/Reader/Xls.php

@@ -2026,9 +2026,9 @@ class Xls extends BaseReader
         $this->pos += 4 + $length;
 
         // offset: 0; size: 2; 0 = base 1900, 1 = base 1904
-        Date::setExcelCalendar(Date::CALENDAR_WINDOWS_1900);
+        $this->spreadsheet->setExcelCalendar(Date::CALENDAR_WINDOWS_1900);
         if (ord($recordData[0]) == 1) {
-            Date::setExcelCalendar(Date::CALENDAR_MAC_1904);
+            $this->spreadsheet->setExcelCalendar(Date::CALENDAR_MAC_1904);
         }
     }
 

src/PhpSpreadsheet/Reader/Xlsx.php

@@ -681,11 +681,11 @@ class Xlsx extends BaseReader
 
                     // Set base date
                     if ($xmlWorkbookNS->workbookPr) {
-                        Date::setExcelCalendar(Date::CALENDAR_WINDOWS_1900);
+                        $excel->setExcelCalendar(Date::CALENDAR_WINDOWS_1900);
                         $attrs1904 = self::getAttributes($xmlWorkbookNS->workbookPr);
                         if (isset($attrs1904['date1904'])) {
                             if (self::boolean((string) $attrs1904['date1904'])) {
-                                Date::setExcelCalendar(Date::CALENDAR_MAC_1904);
+                                $excel->setExcelCalendar(Date::CALENDAR_MAC_1904);
                             }
                         }
                     }

src/PhpSpreadsheet/Shared/Date.php

@@ -67,7 +67,7 @@ class Date
     protected static $defaultTimeZone;
 
     /**
-     * Set the Excel calendar (Windows 1900 or Mac 1904).
+     * Set the Excel Default Calendar (Windows 1900 or Mac 1904).
      *
      * @param int $baseYear Excel base date (1900 or 1904)
      *
@@ -85,7 +85,7 @@ class Date
     }
 
     /**
-     * Return the Excel calendar (Windows 1900 or Mac 1904).
+     * Return the Excel Default Calendar (Windows 1900 or Mac 1904).
      *
      * @return int Excel base date (1900 or 1904)
      */

src/PhpSpreadsheet/Spreadsheet.php

@@ -3,6 +3,7 @@
 namespace PhpOffice\PhpSpreadsheet;
 
 use PhpOffice\PhpSpreadsheet\Calculation\Calculation;
+use PhpOffice\PhpSpreadsheet\Shared\Date;
 use PhpOffice\PhpSpreadsheet\Shared\StringHelper;
 use PhpOffice\PhpSpreadsheet\Style\Style;
 use PhpOffice\PhpSpreadsheet\Worksheet\Iterator;
@@ -52,6 +53,14 @@ class Spreadsheet
      */
     private $workSheetCollection = [];
 
+    /**
+     * Base calendar year to use for calculations
+     * Value is either CALENDAR_WINDOWS_1900 (1900) or CALENDAR_MAC_1904 (1904).
+     *
+     * @var int
+     */
+    protected $excelCalendar = Date::CALENDAR_WINDOWS_1900;
+
     /**
      * Calculation Engine.
      *
@@ -199,6 +208,34 @@ class Spreadsheet
      */
     private $tabRatio = 600;
 
+    /**
+     * Set the Excel Calendar (Windows 1900 or Mac 1904).
+     *
+     * @param int $baseYear Excel base date (1900 or 1904)
+     *
+     * @return bool Success or failure
+     */
+    public function setExcelCalendar(int $baseYear): bool
+    {
+        if (($baseYear == Date::CALENDAR_WINDOWS_1900) || ($baseYear == Date::CALENDAR_MAC_1904)) {
+            $this->excelCalendar = $baseYear;
+
+            return true;
+        }
+
+        return false;
+    }
+
+    /**
+     * Return the Excel Calendar (Windows 1900 or Mac 1904).
+     *
+     * @return int Excel base date (1900 or 1904)
+     */
+    public function getExcelCalendar(): int
+    {
+        return $this->excelCalendar;
+    }
+
     /**
      * The workbook has macros ?
      *

src/PhpSpreadsheet/Writer/Xls/Workbook.php

@@ -960,9 +960,9 @@ class Workbook extends BIFFwriter
         $record = 0x0022; // Record identifier
         $length = 0x0002; // Bytes to follow
 
-        $f1904 = (Date::getExcelCalendar() === Date::CALENDAR_MAC_1904)
-            ? 1
-            : 0; // Flag for 1904 date system
+        $f1904 = ($this->spreadsheet->getExcelCalendar() === Date::CALENDAR_MAC_1904)
+            ? 1  // Flag for 1904 date system
+            : 0; // Flag for 1900 date system
 
         $header = pack('vv', $record, $length);
         $data = pack('v', $f1904);

src/PhpSpreadsheet/Writer/Xlsx/Workbook.php

@@ -39,7 +39,7 @@ class Workbook extends WriterPart
         $this->writeFileVersion($objWriter);
 
         // workbookPr
-        $this->writeWorkbookPr($objWriter);
+        $this->writeWorkbookPr($objWriter, $spreadsheet);
 
         // workbookProtection
         $this->writeWorkbookProtection($objWriter, $spreadsheet);
@@ -80,11 +80,11 @@ class Workbook extends WriterPart
     /**
      * Write WorkbookPr.
      */
-    private function writeWorkbookPr(XMLWriter $objWriter): void
+    private function writeWorkbookPr(XMLWriter $objWriter, Spreadsheet $spreadsheet): void
     {
         $objWriter->startElement('workbookPr');
 
-        if (Date::getExcelCalendar() === Date::CALENDAR_MAC_1904) {
+        if ($spreadsheet->getExcelCalendar() === Date::CALENDAR_MAC_1904) {
             $objWriter->writeAttribute('date1904', '1');
         }