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/RowIterator.php

@@ -8,6 +8,7 @@ use Box\Spout\Reader\IteratorInterface;
 use Box\Spout\Reader\Wrapper\XMLReader;
 use Box\Spout\Reader\XLSX\Helper\CellHelper;
 use Box\Spout\Reader\XLSX\Helper\CellValueFormatter;
+use Box\Spout\Reader\XLSX\Helper\StyleHelper;
 
 /**
  * Class RowIterator
@@ -39,6 +40,9 @@ class RowIterator implements IteratorInterface
     /** @var Helper\CellValueFormatter Helper to format cell values */
     protected $cellValueFormatter;
 
+    /** @var Helper\StyleHelper $styleHelper Helper to work with styles */
+    protected $styleHelper;
+
     /** @var int Number of read rows */
     protected $numReadRows = 0;
 
@@ -62,7 +66,9 @@ class RowIterator implements IteratorInterface
         $this->sheetDataXMLFilePath = $this->normalizeSheetDataXMLFilePath($sheetDataXMLFilePath);
 
         $this->xmlReader = new XMLReader();
-        $this->cellValueFormatter = new CellValueFormatter($sharedStringsHelper);
+
+        $this->styleHelper = new StyleHelper($filePath);
+        $this->cellValueFormatter = new CellValueFormatter($sharedStringsHelper, $this->styleHelper);
     }
 
     /**