diff --git a/data-extra.cabal b/data-extra.cabal
--- a/data-extra.cabal
+++ b/data-extra.cabal
@@ -1,5 +1,5 @@
 name:                data-extra
-version:             2.1.0
+version:             2.1.1
 synopsis:            Extra utilities for working on Data.* types.
 description:         Extra utilities for working on Data.* types.
 license:             BSD3
diff --git a/src/Data/Time/Extra.hs b/src/Data/Time/Extra.hs
--- a/src/Data/Time/Extra.hs
+++ b/src/Data/Time/Extra.hs
@@ -67,12 +67,13 @@
 
 -- | Trivial benchmark for some monadic action.
 bench :: MonadIO m => Integer -> m a -> m (a,NominalDiffTime)
-bench i computation = go i Nothing where
-  go n Nothing = do
+bench i computation = go i (Nothing,0) where
+  go n (Nothing,_) = do
     (a,time) <- stopwatch computation
-    go (n-1) (Just (a,time))
-  go 0 (Just x) = do
-    return x
-  go n (Just (_,avgTime)) = do
-    (a,timeNew) <- stopwatch computation
-    go (n-1) (Just (a,(timeNew + avgTime) / 2))
+    go (n-1) (Just a,time)
+  go 0 (Just x,avg) = do
+    return (x, avg)
+  go n (Just _,avg) = do
+    (a,new) <- stopwatch computation
+    go (n-1) (Just a,(new + avg*cnt) / (cnt+1))
+      where cnt = fromIntegral (i-n)