XLSX cells containing date values should respect shouldFormatDate option (#282)

Return the ISO 8601 date string directly if option is set

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

@@ -283,16 +283,16 @@ class CellValueFormatter
 
     /**
      * Returns a cell's PHP Date value, associated to the given stored nodeValue.
+     * @see ECMA-376 Part 1 - §18.17.4
      *
-     * @param string $nodeValue
-     * @return \DateTime|null The value associated with the cell or NULL if invalid date value
+     * @param string $nodeValue ISO 8601 Date string
+     * @return \DateTime|string|null The value associated with the cell or NULL if invalid date value
      */
     protected function formatDateCellValue($nodeValue)
     {
         // Mitigate thrown Exception on invalid date-time format (http://php.net/manual/en/datetime.construct.php)
         try {
-            $cellValue = new \DateTime($nodeValue);
-            return $cellValue;
+            return ($this->shouldFormatDates) ? $nodeValue : new \DateTime($nodeValue);
         } catch (\Exception $e) {
             return null;
         }