diff --git a/ChangeLog b/ChangeLog
index c2092be4a..3daeb5160 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2012-03-12  Wolfgang Sourdeau  <wsourdeau@inverse.ca>
+
+	* OpenChange/MAPIStoreTypes.m (MAPICNCompare): new function that
+	returns an NSComparisonResult for two change numbers (reverse
+	format).
+
 2012-03-11  Wolfgang Sourdeau  <wsourdeau@inverse.ca>
 
 	* OpenChange/MAPIStoreCalendarMessage.m
diff --git a/OpenChange/MAPIStoreTypes.h b/OpenChange/MAPIStoreTypes.h
index 53fb50272..c630b1e15 100644
--- a/OpenChange/MAPIStoreTypes.h
+++ b/OpenChange/MAPIStoreTypes.h
@@ -42,6 +42,8 @@ id NSObjectFromSPropValue (const struct SPropValue *);
 id NSObjectFromMAPISPropValue (const struct mapi_SPropValue *);
 id NSObjectFromValuePointer (enum MAPITAGS, const void *);
 
+NSComparisonResult MAPICNCompare (uint64_t cn1, uint64_t cn2);
+
 static inline NSNumber *
 MAPIPropertyKey (enum MAPITAGS propTag)
 {
diff --git a/OpenChange/MAPIStoreTypes.m b/OpenChange/MAPIStoreTypes.m
index 56665d0fb..01cd461ea 100644
--- a/OpenChange/MAPIStoreTypes.m
+++ b/OpenChange/MAPIStoreTypes.m
@@ -270,6 +270,34 @@ NSObjectFromValuePointer (enum MAPITAGS propTag, const void *data)
   return result;
 }
 
+static uint64_t
+_reverseCN (uint64_t cn)
+{
+  return ((cn & 0x00000000000000ffL) << 56
+          | (cn & 0x000000000000ff00L) << 40
+          | (cn & 0x0000000000ff0000L) << 24
+          | (cn & 0x00000000ff000000L) << 8
+          | (cn & 0x000000ff00000000L) >> 8
+          | (cn & 0x0000ff0000000000L) >> 24
+          | (cn & 0x00ff000000000000L) >> 40
+          | (cn & 0xff00000000000000L) >> 56);
+}
+
+NSComparisonResult
+MAPICNCompare (uint64_t cn1, uint64_t cn2)
+{
+  NSComparisonResult result;
+
+  if (cn1 == cn2)
+    result = NSOrderedSame;
+  else if (_reverseCN (cn1) < _reverseCN (cn2))
+    result = NSOrderedAscending;
+  else
+    result = NSOrderedDescending;
+
+  return result;
+}
+
 void
 MAPIStoreDumpMessageProperties (NSDictionary *properties)
 {