diff --git a/src/PhpSpreadsheet/Calculation/Calculation.php b/src/PhpSpreadsheet/Calculation/Calculation.php
index eb111437..0d771ff7 100644
--- a/src/PhpSpreadsheet/Calculation/Calculation.php
+++ b/src/PhpSpreadsheet/Calculation/Calculation.php
@@ -4477,7 +4477,7 @@ class Calculation
             }
 
             // if the token is a binary operator, pop the top two values off the stack, do the operation, and push the result back on the stack
-            if (isset(self::$binaryOperators[$token])) {
+            if (!is_numeric($token) && isset(self::$binaryOperators[$token])) {
                 //    We must have two operands, error if we don't
                 if (($operand2Data = $stack->pop()) === null) {
                     return $this->raiseFormulaError('Internal error - Operand value missing from stack');
diff --git a/src/PhpSpreadsheet/Shared/OLE.php b/src/PhpSpreadsheet/Shared/OLE.php
index 8ecfc6be..6278553f 100644
--- a/src/PhpSpreadsheet/Shared/OLE.php
+++ b/src/PhpSpreadsheet/Shared/OLE.php
@@ -502,9 +502,6 @@ class OLE
         }
         $dateTime = Date::dateTimeFromTimestamp("$date");
 
-        // factor used for separating numbers into 4 bytes parts
-        $factor = 2 ** 32;
-
         // days from 1-1-1601 until the beggining of UNIX era
         $days = 134774;
         // calculate seconds
@@ -512,22 +509,15 @@ class OLE
         // multiply just to make MS happy
         $big_date *= 10000000;
 
-        $high_part = floor($big_date / $factor);
-        // lower 4 bytes
-        $low_part = floor((($big_date / $factor) - $high_part) * $factor);
-
         // Make HEX string
         $res = '';
 
-        for ($i = 0; $i < 4; ++$i) {
-            $hex = $low_part % 0x100;
-            $res .= pack('c', $hex);
-            $low_part /= 0x100;
-        }
-        for ($i = 0; $i < 4; ++$i) {
-            $hex = $high_part % 0x100;
-            $res .= pack('c', $hex);
-            $high_part /= 0x100;
+        $factor = 2 ** 56;
+        while ($factor >= 1) {
+            $hex = (int) floor($big_date / $factor);
+            $res = pack('c', $hex) . $res;
+            $big_date = fmod($big_date, $factor);
+            $factor /= 256;
         }
 
         return $res;
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php b/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php
index b0f7d6d4..b73d53cc 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/StringTable.php
@@ -42,8 +42,8 @@ class StringTable extends WriterPart
                 !is_object($cellValue) &&
                 ($cellValue !== null) &&
                 $cellValue !== '' &&
-                !isset($aFlippedStringTable[$cellValue]) &&
-                ($cell->getDataType() == DataType::TYPE_STRING || $cell->getDataType() == DataType::TYPE_STRING2 || $cell->getDataType() == DataType::TYPE_NULL)
+                ($cell->getDataType() == DataType::TYPE_STRING || $cell->getDataType() == DataType::TYPE_STRING2 || $cell->getDataType() == DataType::TYPE_NULL) &&
+                !isset($aFlippedStringTable[$cellValue])
             ) {
                 $aStringTable[] = $cellValue;
                 $aFlippedStringTable[$cellValue] = true;
diff --git a/tests/PhpSpreadsheetTests/Document/EpochTest.php b/tests/PhpSpreadsheetTests/Document/EpochTest.php
index 63c51a59..5ea5c5a8 100644
--- a/tests/PhpSpreadsheetTests/Document/EpochTest.php
+++ b/tests/PhpSpreadsheetTests/Document/EpochTest.php
@@ -11,10 +11,13 @@ class EpochTest extends AbstractFunctional
     public function providerFormats(): array
     {
         return [
+            ['Ods', '1921-03-17 11:30:00Z'],
             ['Ods', '2021-03-17 11:30:00Z'],
             ['Ods', '2041-03-17 11:30:00Z'],
+            ['Xls', '1921-03-17 11:30:00Z'],
             ['Xls', '2021-03-17 11:30:00Z'],
             ['Xls', '2041-03-17 11:30:00Z'],
+            ['Xlsx', '1921-03-17 11:30:00Z'],
             ['Xlsx', '2021-03-17 11:30:00Z'],
             ['Xlsx', '2041-03-17 11:30:00Z'],
         ];