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.

tests/Spout/Reader/XLSX/ReaderTest.php

@@ -113,6 +113,52 @@ class ReaderTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($expectedRows, $allRows);
     }
 
+    /**
+     * @return void
+     */
+    public function testReadShouldSupportNumericTimestampFormattedDifferentlyAsDate()
+    {
+        // make sure dates are always created with the same timezone
+        date_default_timezone_set('UTC');
+
+        $allRows = $this->getAllRowsForFile('sheet_with_same_numeric_value_date_formatted_differently.xlsx');
+
+        $expectedDate = \DateTime::createFromFormat('Y-m-d H:i:s', '2015-01-01 00:00:00');
+        $expectedRows = [
+            array_fill(0, 10, $expectedDate),
+            array_fill(0, 10, $expectedDate),
+            array_fill(0, 10, $expectedDate),
+            array_merge(array_fill(0, 7, $expectedDate), ['', '', '']),
+        ];
+
+        $this->assertEquals($expectedRows, $allRows);
+    }
+
+    /**
+     * @return void
+     */
+    public function testReadShouldSupportDifferentDatesAsNumericTimestamp()
+    {
+        // make sure dates are always created with the same timezone
+        date_default_timezone_set('UTC');
+
+        $allRows = $this->getAllRowsForFile('sheet_with_different_numeric_value_dates.xlsx');
+
+        $expectedRows = [
+            [
+                \DateTime::createFromFormat('Y-m-d H:i:s', '2015-09-01 00:00:00'),
+                \DateTime::createFromFormat('Y-m-d H:i:s', '2015-09-02 00:00:00'),
+                \DateTime::createFromFormat('Y-m-d H:i:s', '2015-09-01 22:23:00'),
+            ],
+            [
+                \DateTime::createFromFormat('Y-m-d H:i:s', '1900-02-28 23:59:59'),
+                \DateTime::createFromFormat('Y-m-d H:i:s', '1900-03-01 00:00:00'),
+                \DateTime::createFromFormat('Y-m-d H:i:s', '1900-02-28 11:00:00'), // 1900-02-29 should be converted to 1900-02-28
+            ]
+        ];
+        $this->assertEquals($expectedRows, $allRows);
+    }
+
     /**
      * @return void
      */