Better date support

Although Excel has a Date type, older Excel versions use numeric values to store dates.
The value represents the number of days since Jan 1st, 1900.
The only way to tell if the value is a number or a date is to look at the styles.xml and check if the cell has date formatting.

src/Spout/Reader/XLSX/Helper/CellValueFormatter.php

@@ -25,19 +25,35 @@ class CellValueFormatter
 
     /** Definition of XML attributes used to parse data */
     const XML_ATTRIBUTE_TYPE = 't';
+    const XML_ATTRIBUTE_STYLE_ID = 's';
+
+    /** Constants used for date formatting */
+    const NUM_DAYS_FROM_JAN_1_1900_TO_JAN_1_1970 = 25569; // actually 25567 but accommodating for an Excel bug with some bisextile years
+    const NUM_SECONDS_IN_ONE_DAY = 86400;
+
+    /**
+     * February 29th, 1900 is NOT a leap year but Excel thinks it is...
+     * @see https://en.wikipedia.org/wiki/Year_1900_problem#Microsoft_Excel
+     */
+    const ERRONEOUS_EXCEL_LEAP_YEAR_DAY = 60;
 
     /** @var SharedStringsHelper Helper to work with shared strings */
     protected $sharedStringsHelper;
 
+    /** @var StyleHelper Helper to work with styles */
+    protected $styleHelper;
+
     /** @var \Box\Spout\Common\Escaper\XLSX Used to unescape XML data */
     protected $escaper;
 
     /**
      * @param SharedStringsHelper $sharedStringsHelper Helper to work with shared strings
+     * @param StyleHelper $styleHelper Helper to work with styles
      */
-    public function __construct($sharedStringsHelper)
+    public function __construct($sharedStringsHelper, $styleHelper)
     {
         $this->sharedStringsHelper = $sharedStringsHelper;
+        $this->styleHelper = $styleHelper;
 
         /** @noinspection PhpUnnecessaryFullyQualifiedNameInspection */
         $this->escaper = new \Box\Spout\Common\Escaper\XLSX();
@@ -53,6 +69,7 @@ class CellValueFormatter
     {
         // Default cell type is "n"
         $cellType = $node->getAttribute(self::XML_ATTRIBUTE_TYPE) ?: self::CELL_TYPE_NUMERIC;
+        $cellStyleId = intval($node->getAttribute(self::XML_ATTRIBUTE_STYLE_ID));
         $vNodeValue = $this->getVNodeValue($node);
 
         if (($vNodeValue === '') && ($cellType !== self::CELL_TYPE_INLINE_STRING)) {
@@ -69,7 +86,7 @@ class CellValueFormatter
             case self::CELL_TYPE_BOOLEAN:
                 return $this->formatBooleanCellValue($vNodeValue);
             case self::CELL_TYPE_NUMERIC:
-                return $this->formatNumericCellValue($vNodeValue);
+                return $this->formatNumericCellValue($vNodeValue, $cellStyleId);
             case self::CELL_TYPE_DATE:
                 return $this->formatDateCellValue($vNodeValue);
             default:
@@ -138,14 +155,49 @@ class CellValueFormatter
 
     /**
      * Returns the cell Numeric value from string of nodeValue.
+     * The value can also represent a timestamp and a DateTime will be returned.
      *
      * @param string $nodeValue
-     * @return int|float The value associated with the cell
+     * @param int $cellStyleId 0 being the default style
+     * @return int|float|\DateTime|null The value associated with the cell
      */
-    protected function formatNumericCellValue($nodeValue)
+    protected function formatNumericCellValue($nodeValue, $cellStyleId)
     {
-        $cellValue = is_int($nodeValue) ? intval($nodeValue) : floatval($nodeValue);
-        return $cellValue;
+        // Numeric values can represent numbers as well as timestamps.
+        // We need to look at the style of the cell to determine whether it is one or the other.
+        $shouldFormatAsDate = $this->styleHelper->shouldFormatNumericValueAsDate($cellStyleId);
+
+        if ($shouldFormatAsDate) {
+            return $this->formatExcelTimestampValue(floatval($nodeValue));
+        } else {
+            return is_int($nodeValue) ? intval($nodeValue) : floatval($nodeValue);
+        }
+    }
+
+    /**
+     * Returns a cell's PHP Date value, associated to the given timestamp.
+     * NOTE: The timestamp is a float representing the number of days since January 1st, 1900.
+     *
+     * @param float $nodeValue
+     * @return \DateTime|null The value associated with the cell or NULL if invalid date value
+     */
+    protected function formatExcelTimestampValue($nodeValue)
+    {
+        $numDaysSince1970Jan1 = $nodeValue - self::NUM_DAYS_FROM_JAN_1_1900_TO_JAN_1_1970;
+
+        // Fix for the erroneous leap year in Excel
+        if ($nodeValue < self::ERRONEOUS_EXCEL_LEAP_YEAR_DAY) {
+            $numDaysSince1970Jan1++;
+        }
+
+        $unixTimestamp = round($numDaysSince1970Jan1 * self::NUM_SECONDS_IN_ONE_DAY);
+
+        try {
+            $cellValue = (new \DateTime())->setTimestamp($unixTimestamp);
+            return $cellValue;
+        } catch (\Exception $e) {
+            return null;
+        }
     }
 
     /**