diff --git a/CHANGELOG.md b/CHANGELOG.md
index c8612cad6c..229a320227 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 ### Changed
 
 - On read, Xlsx Reader had been breaking up union ranges into separate individual ranges. It will now try to preserve range as it was read in. [PR #4042](https://github.com/PHPOffice/PhpSpreadsheet/pull/4042)
+- Xlsx/Xls spreadsheet calculation and formatting of dates will use base date of spreadsheet even when spreadsheets with different base dates are simultaneously open. [Issue #1036](https://github.com/PHPOffice/PhpSpreadsheet/issues/1036) [Issue #1635](https://github.com/PHPOffice/PhpSpreadsheet/issues/1635) [PR #4071](https://github.com/PHPOffice/PhpSpreadsheet/pull/4071) 
 
 ### Deprecated
 
diff --git a/src/PhpSpreadsheet/Cell/Cell.php b/src/PhpSpreadsheet/Cell/Cell.php
index 03fa29c545..0f0075bcaa 100644
--- a/src/PhpSpreadsheet/Cell/Cell.php
+++ b/src/PhpSpreadsheet/Cell/Cell.php
@@ -187,10 +187,15 @@ public function getValueString(): string
      */
     public function getFormattedValue(): string
     {
-        return (string) NumberFormat::toFormattedString(
-            $this->getCalculatedValueString(),
+        $currentCalendar = SharedDate::getExcelCalendar();
+        SharedDate::setExcelCalendar($this->getWorksheet()->getParent()?->getExcelCalendar());
+        $formattedValue = (string) NumberFormat::toFormattedString(
+            $this->getCalculatedValue(),
             (string) $this->getStyle()->getNumberFormat()->getFormatCode(true)
         );
+        SharedDate::setExcelCalendar($currentCalendar);
+
+        return $formattedValue;
     }
 
     protected static function updateIfCellIsTableHeader(?Worksheet $workSheet, self $cell, mixed $oldValue, mixed $newValue): void
@@ -372,6 +377,8 @@ public function getCalculatedValue(bool $resetLog = true): mixed
     {
         if ($this->dataType === DataType::TYPE_FORMULA) {
             try {
+                $currentCalendar = SharedDate::getExcelCalendar();
+                SharedDate::setExcelCalendar($this->getWorksheet()->getParent()?->getExcelCalendar());
                 $index = $this->getWorksheet()->getParentOrThrow()->getActiveSheetIndex();
                 $selected = $this->getWorksheet()->getSelectedCells();
                 $result = Calculation::getInstance(
@@ -387,6 +394,7 @@ public function getCalculatedValue(bool $resetLog = true): mixed
                     }
                 }
             } catch (SpreadsheetException $ex) {
+                SharedDate::setExcelCalendar($currentCalendar);
                 if (($ex->getMessage() === 'Unable to access External Workbook') && ($this->calculatedValue !== null)) {
                     return $this->calculatedValue; // Fallback for calculations referencing external files.
                 } elseif (preg_match('/[Uu]ndefined (name|offset: 2|array key 2)/', $ex->getMessage()) === 1) {
@@ -399,6 +407,7 @@ public function getCalculatedValue(bool $resetLog = true): mixed
                     $ex
                 );
             }
+            SharedDate::setExcelCalendar($currentCalendar);
 
             if ($result === '#Not Yet Implemented') {
                 return $this->calculatedValue; // Fallback if calculation engine does not support the formula.
diff --git a/src/PhpSpreadsheet/Reader/Xls.php b/src/PhpSpreadsheet/Reader/Xls.php
index 58a22835de..be9ccb2681 100644
--- a/src/PhpSpreadsheet/Reader/Xls.php
+++ b/src/PhpSpreadsheet/Reader/Xls.php
@@ -1927,8 +1927,10 @@ private function readDateMode(): void
 
         // 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);
         }
     }
 
diff --git a/src/PhpSpreadsheet/Reader/Xlsx.php b/src/PhpSpreadsheet/Reader/Xlsx.php
index 9fdf96aeca..882f29644f 100644
--- a/src/PhpSpreadsheet/Reader/Xlsx.php
+++ b/src/PhpSpreadsheet/Reader/Xlsx.php
@@ -712,12 +712,14 @@ protected function loadSpreadsheetFromFile(string $filename): Spreadsheet
                     $xmlWorkbookNS = $this->loadZip($relTarget, $mainNS);
 
                     // Set base date
+                    $excel->setExcelCalendar(Date::CALENDAR_WINDOWS_1900);
                     if ($xmlWorkbookNS->workbookPr) {
                         Date::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);
                             }
                         }
                     }
diff --git a/src/PhpSpreadsheet/Shared/Date.php b/src/PhpSpreadsheet/Shared/Date.php
index ed19534de0..b8feeb9c20 100644
--- a/src/PhpSpreadsheet/Shared/Date.php
+++ b/src/PhpSpreadsheet/Shared/Date.php
@@ -10,7 +10,6 @@
 use PhpOffice\PhpSpreadsheet\Cell\Cell;
 use PhpOffice\PhpSpreadsheet\Exception;
 use PhpOffice\PhpSpreadsheet\Exception as PhpSpreadsheetException;
-use PhpOffice\PhpSpreadsheet\Shared\Date as SharedDate;
 use PhpOffice\PhpSpreadsheet\Style\NumberFormat;
 
 class Date
@@ -64,15 +63,15 @@ class Date
     /**
      * Set the Excel calendar (Windows 1900 or Mac 1904).
      *
-     * @param int $baseYear Excel base date (1900 or 1904)
+     * @param ?int $baseYear Excel base date (1900 or 1904)
      *
      * @return bool Success or failure
      */
-    public static function setExcelCalendar(int $baseYear): bool
+    public static function setExcelCalendar(?int $baseYear): bool
     {
         if (
-            ($baseYear == self::CALENDAR_WINDOWS_1900)
-            || ($baseYear == self::CALENDAR_MAC_1904)
+            ($baseYear === self::CALENDAR_WINDOWS_1900)
+            || ($baseYear === self::CALENDAR_MAC_1904)
         ) {
             self::$excelCalendar = $baseYear;
 
@@ -173,7 +172,7 @@ public static function convertIsoDate(mixed $value): float|int
             throw new Exception("Invalid string $value supplied for datatype Date");
         }
 
-        $newValue = SharedDate::PHPToExcel($date);
+        $newValue = self::PHPToExcel($date);
         if ($newValue === false) {
             throw new Exception("Invalid string $value supplied for datatype Date");
         }
diff --git a/src/PhpSpreadsheet/Spreadsheet.php b/src/PhpSpreadsheet/Spreadsheet.php
index 095507d7b7..e571cc4f6b 100644
--- a/src/PhpSpreadsheet/Spreadsheet.php
+++ b/src/PhpSpreadsheet/Spreadsheet.php
@@ -7,6 +7,7 @@
 use PhpOffice\PhpSpreadsheet\Document\Properties;
 use PhpOffice\PhpSpreadsheet\Document\Security;
 use PhpOffice\PhpSpreadsheet\Reader\Xlsx as XlsxReader;
+use PhpOffice\PhpSpreadsheet\Shared\Date;
 use PhpOffice\PhpSpreadsheet\Shared\File;
 use PhpOffice\PhpSpreadsheet\Shared\StringHelper;
 use PhpOffice\PhpSpreadsheet\Style\Style;
@@ -31,6 +32,8 @@ class Spreadsheet implements JsonSerializable
         self::VISIBILITY_VERY_HIDDEN,
     ];
 
+    protected int $excelCalendar = Date::CALENDAR_WINDOWS_1900;
+
     /**
      * Unique ID.
      */
@@ -1553,4 +1556,26 @@ public function getTableByName(string $tableName): ?Table
 
         return $table;
     }
+
+    /**
+     * @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 int Excel base date (1900 or 1904)
+     */
+    public function getExcelCalendar(): int
+    {
+        return $this->excelCalendar;
+    }
 }
diff --git a/src/PhpSpreadsheet/Writer/Xls/Workbook.php b/src/PhpSpreadsheet/Writer/Xls/Workbook.php
index 62d4b08d9d..a1a5faf8f4 100644
--- a/src/PhpSpreadsheet/Writer/Xls/Workbook.php
+++ b/src/PhpSpreadsheet/Writer/Xls/Workbook.php
@@ -910,9 +910,9 @@ private function writeDateMode(): void
         $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);
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php b/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php
index 5604be5443..0cdfb3cd87 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Workbook.php
@@ -40,7 +40,7 @@ public function writeWorkbook(Spreadsheet $spreadsheet, bool $recalcRequired = f
         $this->writeFileVersion($objWriter);
 
         // workbookPr
-        $this->writeWorkbookPr($objWriter);
+        $this->writeWorkbookPr($objWriter, $spreadsheet);
 
         // workbookProtection
         $this->writeWorkbookProtection($objWriter, $spreadsheet);
@@ -81,11 +81,11 @@ private function writeFileVersion(XMLWriter $objWriter): void
     /**
      * 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');
         }
 
diff --git a/tests/PhpSpreadsheetTests/Reader/Xls/DateReaderTest.php b/tests/PhpSpreadsheetTests/Reader/Xls/DateReaderTest.php
new file mode 100644
index 0000000000..58bcbb95b4
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Reader/Xls/DateReaderTest.php
@@ -0,0 +1,127 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheetTests\Reader\Xls;
+
+use PhpOffice\PhpSpreadsheet\Reader\Xls;
+use PhpOffice\PhpSpreadsheet\Shared\Date;
+use PHPUnit\Framework\TestCase;
+
+class DateReaderTest extends TestCase
+{
+    protected function tearDown(): void
+    {
+        Date::setExcelCalendar(Date::CALENDAR_WINDOWS_1900);
+    }
+
+    public function testReadExcel1900Spreadsheet(): void
+    {
+        $filename = 'tests/data/Reader/XLS/1900_Calendar.xls';
+        $reader = new Xls();
+        $spreadsheet = $reader->load($filename);
+
+        self::assertSame(Date::CALENDAR_WINDOWS_1900, $spreadsheet->getExcelCalendar());
+
+        $worksheet = $spreadsheet->getActiveSheet();
+        self::assertSame(44562, $worksheet->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet->getCell('A1')->getFormattedValue());
+        self::assertSame(44926, $worksheet->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet->getCell('A2')->getFormattedValue());
+        $spreadsheet->disconnectWorksheets();
+    }
+
+    public function testReadExcel1904Spreadsheet(): void
+    {
+        $filename = 'tests/data/Reader/XLS/1904_Calendar.xls';
+        $reader = new Xls();
+        $spreadsheet = $reader->load($filename);
+
+        self::assertSame(Date::CALENDAR_MAC_1904, $spreadsheet->getExcelCalendar());
+
+        $worksheet = $spreadsheet->getActiveSheet();
+        self::assertSame(43100, $worksheet->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet->getCell('A1')->getFormattedValue());
+        self::assertSame(43464, $worksheet->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet->getCell('A2')->getFormattedValue());
+        $spreadsheet->disconnectWorksheets();
+    }
+
+    public function testNewDateInLoadedExcel1900Spreadsheet(): void
+    {
+        $filename = 'tests/data/Reader/XLS/1900_Calendar.xls';
+        $reader = new Xls();
+        $spreadsheet = $reader->load($filename);
+
+        $worksheet = $spreadsheet->getActiveSheet();
+        $worksheet->getCell('A4')->setValue('=DATE(2023,1,1)');
+        self::assertEquals(44927, $worksheet->getCell('A4')->getCalculatedValue());
+        $spreadsheet->disconnectWorksheets();
+    }
+
+    public function testNewDateInLoadedExcel1904Spreadsheet(): void
+    {
+        $filename = 'tests/data/Reader/XLS/1904_Calendar.xls';
+        $reader = new Xls();
+        $spreadsheet = $reader->load($filename);
+
+        $worksheet = $spreadsheet->getActiveSheet();
+        $worksheet->getCell('A4')->setValue('=DATE(2023,1,1)');
+        self::assertEquals(43465, $worksheet->getCell('A4')->getCalculatedValue());
+        $spreadsheet->disconnectWorksheets();
+    }
+
+    public function testSwitchCalendars(): void
+    {
+        $filename1904 = 'tests/data/Reader/XLS/1904_Calendar.xls';
+        $reader1904 = new Xls();
+        $spreadsheet1904 = $reader1904->load($filename1904);
+        $worksheet1904 = $spreadsheet1904->getActiveSheet();
+        $date1 = Date::convertIsoDate('2022-01-01');
+        self::assertSame(43100.0, $date1);
+
+        $filename1900 = 'tests/data/Reader/XLS/1900_Calendar.xls';
+        $reader1900 = new Xls();
+        $spreadsheet1900 = $reader1900->load($filename1900);
+        $worksheet1900 = $spreadsheet1900->getActiveSheet();
+        $date2 = Date::convertIsoDate('2022-01-01');
+        self::assertSame(44562.0, $date2);
+
+        self::assertSame(44562, $worksheet1900->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet1900->getCell('A1')->getFormattedValue());
+        self::assertSame(44926, $worksheet1900->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet1900->getCell('A2')->getFormattedValue());
+        self::assertSame(44561, $worksheet1900->getCell('B1')->getCalculatedValue());
+        self::assertSame('2021-12-31', $worksheet1900->getCell('B1')->getFormattedValue());
+        self::assertSame(44927, $worksheet1900->getCell('B2')->getCalculatedValue());
+        self::assertSame('2023-01-01', $worksheet1900->getCell('B2')->getFormattedValue());
+
+        self::assertSame(43100, $worksheet1904->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet1904->getCell('A1')->getFormattedValue());
+        self::assertSame(43464, $worksheet1904->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet1904->getCell('A2')->getFormattedValue());
+        self::assertSame(43099, $worksheet1904->getCell('B1')->getCalculatedValue());
+        self::assertSame('2021-12-31', $worksheet1904->getCell('B1')->getFormattedValue());
+        self::assertSame(43465, $worksheet1904->getCell('B2')->getCalculatedValue());
+        self::assertSame('2023-01-01', $worksheet1904->getCell('B2')->getFormattedValue());
+
+        // Check that accessing date values from one spreadsheet doesn't break accessing correct values from another
+        self::assertSame(44561, $worksheet1900->getCell('B1')->getCalculatedValue());
+        self::assertSame('2021-12-31', $worksheet1900->getCell('B1')->getFormattedValue());
+        self::assertSame(44927, $worksheet1900->getCell('B2')->getCalculatedValue());
+        self::assertSame('2023-01-01', $worksheet1900->getCell('B2')->getFormattedValue());
+        self::assertSame(44562, $worksheet1900->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet1900->getCell('A1')->getFormattedValue());
+        self::assertSame(44926, $worksheet1900->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet1900->getCell('A2')->getFormattedValue());
+
+        self::assertSame(43099, $worksheet1904->getCell('B1')->getCalculatedValue());
+        self::assertSame('2021-12-31', $worksheet1904->getCell('B1')->getFormattedValue());
+        self::assertSame(43465, $worksheet1904->getCell('B2')->getCalculatedValue());
+        self::assertSame('2023-01-01', $worksheet1904->getCell('B2')->getFormattedValue());
+        self::assertSame(43100, $worksheet1904->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet1904->getCell('A1')->getFormattedValue());
+        self::assertSame(43464, $worksheet1904->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet1904->getCell('A2')->getFormattedValue());
+        $spreadsheet1900->disconnectWorksheets();
+        $spreadsheet1904->disconnectWorksheets();
+    }
+}
diff --git a/tests/PhpSpreadsheetTests/Reader/Xlsx/DateReaderTest.php b/tests/PhpSpreadsheetTests/Reader/Xlsx/DateReaderTest.php
new file mode 100644
index 0000000000..7eaee0df73
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Reader/Xlsx/DateReaderTest.php
@@ -0,0 +1,135 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheetTests\Reader\Xlsx;
+
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx;
+use PhpOffice\PhpSpreadsheet\Shared\Date;
+use PHPUnit\Framework\TestCase;
+
+class DateReaderTest extends TestCase
+{
+    protected function tearDown(): void
+    {
+        Date::setExcelCalendar(Date::CALENDAR_WINDOWS_1900);
+    }
+
+    public function testReadExcel1900Spreadsheet(): void
+    {
+        $filename = 'tests/data/Reader/XLSX/1900_Calendar.xlsx';
+        $reader = new Xlsx();
+        $spreadsheet = $reader->load($filename);
+
+        self::assertSame(Date::CALENDAR_WINDOWS_1900, $spreadsheet->getExcelCalendar());
+
+        $worksheet = $spreadsheet->getActiveSheet();
+        self::assertSame(44562, $worksheet->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet->getCell('A1')->getFormattedValue());
+        self::assertSame(44926, $worksheet->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet->getCell('A2')->getFormattedValue());
+        self::assertSame(44561, $worksheet->getCell('B1')->getCalculatedValue());
+        self::assertSame('2021-12-31', $worksheet->getCell('B1')->getFormattedValue());
+        self::assertSame(44927, $worksheet->getCell('B2')->getCalculatedValue());
+        self::assertSame('2023-01-01', $worksheet->getCell('B2')->getFormattedValue());
+        $spreadsheet->disconnectWorksheets();
+    }
+
+    public function testReadExcel1904Spreadsheet(): void
+    {
+        $filename = 'tests/data/Reader/XLSX/1904_Calendar.xlsx';
+        $reader = new Xlsx();
+        $spreadsheet = $reader->load($filename);
+
+        self::assertSame(Date::CALENDAR_MAC_1904, $spreadsheet->getExcelCalendar());
+
+        $worksheet = $spreadsheet->getActiveSheet();
+        self::assertSame(43100, $worksheet->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet->getCell('A1')->getFormattedValue());
+        self::assertSame(43464, $worksheet->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet->getCell('A2')->getFormattedValue());
+        self::assertSame(43099, $worksheet->getCell('B1')->getCalculatedValue());
+        self::assertSame('2021-12-31', $worksheet->getCell('B1')->getFormattedValue());
+        self::assertSame(43465, $worksheet->getCell('B2')->getCalculatedValue());
+        self::assertSame('2023-01-01', $worksheet->getCell('B2')->getFormattedValue());
+        $spreadsheet->disconnectWorksheets();
+    }
+
+    public function testNewDateInLoadedExcel1900Spreadsheet(): void
+    {
+        $filename = 'tests/data/Reader/XLSX/1900_Calendar.xlsx';
+        $reader = new Xlsx();
+        $spreadsheet = $reader->load($filename);
+
+        $worksheet = $spreadsheet->getActiveSheet();
+        $worksheet->getCell('A4')->setValue('=DATE(2023,1,1)');
+        self::assertEquals(44927, $worksheet->getCell('A4')->getCalculatedValue());
+        $spreadsheet->disconnectWorksheets();
+    }
+
+    public function testNewDateInLoadedExcel1904Spreadsheet(): void
+    {
+        $filename = 'tests/data/Reader/XLSX/1904_Calendar.xlsx';
+        $reader = new Xlsx();
+        $spreadsheet = $reader->load($filename);
+
+        $worksheet = $spreadsheet->getActiveSheet();
+        $worksheet->getCell('A4')->setValue('=DATE(2023,1,1)');
+        self::assertEquals(43465, $worksheet->getCell('A4')->getCalculatedValue());
+        $spreadsheet->disconnectWorksheets();
+    }
+
+    public function testSwitchCalendars(): void
+    {
+        $filename1904 = 'tests/data/Reader/XLSX/1904_Calendar.xlsx';
+        $reader1904 = new Xlsx();
+        $spreadsheet1904 = $reader1904->load($filename1904);
+        $worksheet1904 = $spreadsheet1904->getActiveSheet();
+        $date1 = Date::convertIsoDate('2022-01-01');
+        self::assertSame(43100.0, $date1);
+
+        $filename1900 = 'tests/data/Reader/XLSX/1900_Calendar.xlsx';
+        $reader1900 = new Xlsx();
+        $spreadsheet1900 = $reader1900->load($filename1900);
+        $worksheet1900 = $spreadsheet1900->getActiveSheet();
+        $date2 = Date::convertIsoDate('2022-01-01');
+        self::assertSame(44562.0, $date2);
+
+        self::assertSame(44562, $worksheet1900->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet1900->getCell('A1')->getFormattedValue());
+        self::assertSame(44926, $worksheet1900->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet1900->getCell('A2')->getFormattedValue());
+        self::assertSame(44561, $worksheet1900->getCell('B1')->getCalculatedValue());
+        self::assertSame('2021-12-31', $worksheet1900->getCell('B1')->getFormattedValue());
+        self::assertSame(44927, $worksheet1900->getCell('B2')->getCalculatedValue());
+        self::assertSame('2023-01-01', $worksheet1900->getCell('B2')->getFormattedValue());
+
+        self::assertSame(43100, $worksheet1904->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet1904->getCell('A1')->getFormattedValue());
+        self::assertSame(43464, $worksheet1904->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet1904->getCell('A2')->getFormattedValue());
+        self::assertSame(43099, $worksheet1904->getCell('B1')->getCalculatedValue());
+        self::assertSame('2021-12-31', $worksheet1904->getCell('B1')->getFormattedValue());
+        self::assertSame(43465, $worksheet1904->getCell('B2')->getCalculatedValue());
+        self::assertSame('2023-01-01', $worksheet1904->getCell('B2')->getFormattedValue());
+
+        // Check that accessing date values from one spreadsheet doesn't break accessing correct values from another
+        self::assertSame(44561, $worksheet1900->getCell('B1')->getCalculatedValue());
+        self::assertSame('2021-12-31', $worksheet1900->getCell('B1')->getFormattedValue());
+        self::assertSame(44927, $worksheet1900->getCell('B2')->getCalculatedValue());
+        self::assertSame('2023-01-01', $worksheet1900->getCell('B2')->getFormattedValue());
+        self::assertSame(44562, $worksheet1900->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet1900->getCell('A1')->getFormattedValue());
+        self::assertSame(44926, $worksheet1900->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet1900->getCell('A2')->getFormattedValue());
+
+        self::assertSame(43099, $worksheet1904->getCell('B1')->getCalculatedValue());
+        self::assertSame('2021-12-31', $worksheet1904->getCell('B1')->getFormattedValue());
+        self::assertSame(43465, $worksheet1904->getCell('B2')->getCalculatedValue());
+        self::assertSame('2023-01-01', $worksheet1904->getCell('B2')->getFormattedValue());
+        self::assertSame(43100, $worksheet1904->getCell('A1')->getValue());
+        self::assertSame('2022-01-01', $worksheet1904->getCell('A1')->getFormattedValue());
+        self::assertSame(43464, $worksheet1904->getCell('A2')->getValue());
+        self::assertSame('2022-12-31', $worksheet1904->getCell('A2')->getFormattedValue());
+        $spreadsheet1900->disconnectWorksheets();
+        $spreadsheet1904->disconnectWorksheets();
+    }
+}
diff --git a/tests/data/Reader/XLS/1900_Calendar.xls b/tests/data/Reader/XLS/1900_Calendar.xls
new file mode 100644
index 0000000000..714670a7dc
Binary files /dev/null and b/tests/data/Reader/XLS/1900_Calendar.xls differ
diff --git a/tests/data/Reader/XLS/1904_Calendar.xls b/tests/data/Reader/XLS/1904_Calendar.xls
new file mode 100644
index 0000000000..cde7fc1931
Binary files /dev/null and b/tests/data/Reader/XLS/1904_Calendar.xls differ
diff --git a/tests/data/Reader/XLSX/1900_Calendar.xlsx b/tests/data/Reader/XLSX/1900_Calendar.xlsx
new file mode 100644
index 0000000000..dfd78efc7c
Binary files /dev/null and b/tests/data/Reader/XLSX/1900_Calendar.xlsx differ
diff --git a/tests/data/Reader/XLSX/1904_Calendar.xlsx b/tests/data/Reader/XLSX/1904_Calendar.xlsx
new file mode 100644
index 0000000000..f3a8a12d8e
Binary files /dev/null and b/tests/data/Reader/XLSX/1904_Calendar.xlsx differ