diff --git a/Spreadsheet/Excel/Writer/Workbook.php b/Spreadsheet/Excel/Writer/Workbook.php
index a1c97f3..788d461 100644
--- a/Spreadsheet/Excel/Writer/Workbook.php
+++ b/Spreadsheet/Excel/Writer/Workbook.php
@@ -166,7 +166,7 @@ class Spreadsheet_Excel_Writer_Workbook extends Spreadsheet_Excel_Writer_BIFFwri
     var $_string_sizeinfo_size;
 
     /** @var int */
-    public $timestamp;
+    var $_timestamp;
 
     /**
     * Class constructor
@@ -203,7 +203,7 @@ class Spreadsheet_Excel_Writer_Workbook extends Spreadsheet_Excel_Writer_BIFFwri
         $this->_str_total       = 0;
         $this->_str_unique      = 0;
         $this->_str_table       = array();
-        $this->timestamp            = time();
+        $this->_timestamp       = time();
 
         $this->_setPaletteXl97();
     }
@@ -595,7 +595,7 @@ class Spreadsheet_Excel_Writer_Workbook extends Spreadsheet_Excel_Writer_BIFFwri
             }
         }
 
-        $root = new OLE_PPS_Root($this->timestamp, $this->timestamp, array($OLE));
+        $root = new OLE_PPS_Root($this->_timestamp, $this->_timestamp, array($OLE));
         if ($this->_tmp_dir != '') {
             $root->setTempDir($this->_tmp_dir);
         }
diff --git a/test/Test/Spreadsheet/Excel/Writer/Test_Spreadsheet_Excel_WriterTestCase.php b/test/Test/Spreadsheet/Excel/Writer/Test_Spreadsheet_Excel_WriterTestCase.php
index 00faa30..7d7d2a8 100644
--- a/test/Test/Spreadsheet/Excel/Writer/Test_Spreadsheet_Excel_WriterTestCase.php
+++ b/test/Test/Spreadsheet/Excel/Writer/Test_Spreadsheet_Excel_WriterTestCase.php
@@ -20,10 +20,10 @@ class Test_Spreadsheet_Excel_WriterTestCase extends PHPUnit_Framework_TestCase
 
     protected function assertSameAsInFixture($filename, Spreadsheet_Excel_Writer $workbook)
     {
-        $this->assertEmpty($workbook->fileName, "Testing with fixtures works only for standard output");
+        $this->assertEmpty($workbook->_filename, "Testing with fixtures works only for standard output");
 
         // we have to fix timestamp for fixtures to work
-        $workbook->timestamp = 1000000000; // somewhere in 2001
+        $workbook->_timestamp = 1000000000; // somewhere in 2001
 
         ob_start();
         $workbook->close();