--- poi-3.9/src/java/org/apache/poi/hssf/usermodel/HSSFCellStyle.java	2012-11-25 13:27:35.000000000 +0100
+++ poi-3.9/src/java/org/apache/poi/hssf/usermodel/HSSFCellStyle.java	2013-10-22 11:18:32.000000000 +0200
@@ -18,9 +18,13 @@ 
 
 package org.apache.poi.hssf.usermodel;
 
+import java.util.Arrays;
+import java.util.List;
+
 import org.apache.poi.hssf.model.InternalWorkbook;
 import org.apache.poi.hssf.record.ExtendedFormatRecord;
 import org.apache.poi.hssf.record.FontRecord;
+import org.apache.poi.hssf.record.FormatRecord;
 import org.apache.poi.hssf.record.StyleRecord;
 import org.apache.poi.hssf.util.HSSFColor;
 import org.apache.poi.ss.usermodel.CellStyle;
@@ -106,8 +110,42 @@ public final class HSSFCellStyle impleme
      * @see org.apache.poi.hssf.usermodel.HSSFDataFormat
      * @return the format string or "General" if not found
      */
+         
+    private static ThreadLocal<Short> lastDateFormat = new ThreadLocal<Short>() {
+      @Override
+      protected Short initialValue() {
+         return Short.MIN_VALUE;
+      }
+    };
+    
+    private static ThreadLocal<List<FormatRecord>> lastFormats = new ThreadLocal<List<FormatRecord>>() {
+      @Override
+      protected List<FormatRecord> initialValue() {
+         return null;
+      }
+    };
+
+    private static ThreadLocal<String> getDataFormatStringCache = new ThreadLocal<String>() {
+      @Override
+      protected String initialValue() {
+         return null;
+      }
+    };
+
     public String getDataFormatString() {
-        return getDataFormatString(_workbook);
+      if (getDataFormatStringCache.get() != null) {
+         if (lastDateFormat.get() == getDataFormat() && _workbook.getFormats().equals(lastFormats.get())) {
+            return getDataFormatStringCache.get();
+         }
+      }
+      
+      lastFormats.set(_workbook.getFormats());
+      lastDateFormat.set(getDataFormat());
+      
+      getDataFormatStringCache.set(getDataFormatString(_workbook));
+      
+      return getDataFormatStringCache.get();
+
     }
     /**
      * Get the contents of the format string, by looking up
@@ -828,6 +866,11 @@ public final class HSSFCellStyle impleme
 
     	// Handle matching things if we cross workbooks
     	if(_workbook != source._workbook) {
+    	
+    	   lastDateFormat.set(Short.MIN_VALUE);
+           lastFormats.set(null);
+           getDataFormatStringCache.set(null);
+    	   
 			// Then we need to clone the format string,
 			//  and update the format record for this
     		short fmt = (short)_workbook.createFormat(source.getDataFormatString() );
@@ -874,4 +917,5 @@ public final class HSSFCellStyle impleme
 		}
 		return false;
 	}
+	
 }