--- poi-3.9/src/java/org/apache/poi/ss/usermodel/DateUtil.java	2012-11-25 13:28:05.000000000 +0100
+++ poi-3.9/src/java/org/apache/poi/ss/usermodel/DateUtil.java	2013-10-19 17:19:57.000000000 +0200
@@ -288,17 +288,31 @@ public class DateUtil {
      * @param formatIndex The index of the format, eg from ExtendedFormatRecord.getFormatIndex
      * @param formatString The format string, eg from FormatRecord.getFormatString
      * @see #isInternalDateFormat(int)
-     */
-    public static boolean isADateFormat(int formatIndex, String formatString) {
-        // First up, is this an internal date format?
-        if(isInternalDateFormat(formatIndex)) {
-            return true;
-        }
-
-        // If we didn't get a real string, it can't be
-        if(formatString == null || formatString.length() == 0) {
-            return false;
-        }
+     */  
+    private static int lastFormatIndex = Integer.MIN_VALUE;
+    private static String lastFormatString = null;
+    private static boolean isADateFormatCache = false;
+    
+    public static synchronized boolean isADateFormat(int formatIndex, String formatString) {
+       
+       if (formatIndex == lastFormatIndex && formatString != null && formatString.equals(lastFormatString)) {
+          return isADateFormatCache;
+       }
+       
+       lastFormatIndex = formatIndex;
+       lastFormatString = formatString;
+       
+       // First up, is this an internal date format?
+       if (isInternalDateFormat(formatIndex)) {
+          isADateFormatCache = true;
+          return true;
+       }
+
+       // If we didn't get a real string, it can't be
+       if (formatString == null || formatString.length() == 0) {
+          isADateFormatCache = false;
+          return false;
+       }
 
         String fs = formatString;
         /*if (false) {
@@ -348,6 +362,7 @@ public class DateUtil {
 
         // short-circuit if it indicates elapsed time: [h], [m] or [s]
         if(date_ptrn4.matcher(fs).matches()){
+            isADateFormatCache = true;
             return true;
         }
 
@@ -367,7 +382,9 @@ public class DateUtil {
         // Otherwise, check it's only made up, in any case, of:
         //  y m d h s - \ / , . :
         // optionally followed by AM/PM
-        return date_ptrn3.matcher(fs).matches();
+        boolean result = date_ptrn3.matcher(fs).matches();
+        isADateFormatCache = result;
+        return result;
     }
 
     /**