diff --git a/library/PostgreSQL/Binary/Decoding.hs b/library/PostgreSQL/Binary/Decoding.hs
--- a/library/PostgreSQL/Binary/Decoding.hs
+++ b/library/PostgreSQL/Binary/Decoding.hs
@@ -51,6 +51,7 @@
   hstore,
   -- **
   enum,
+  refine,
 )
 where
 
@@ -537,3 +538,13 @@
           failure ("No mapping for text \"" <> text <> "\"")
         onJust =
           pure
+
+-- * Refining values
+-------------------------
+
+-- | Given additional constraints when
+-- using an existing value decoder, produces
+-- a decoder of that value.
+{-# INLINE refine #-}
+refine :: (a -> Either Text b) -> Value a -> Value b
+refine fn m = m >>= (either failure pure . fn)
diff --git a/postgresql-binary.cabal b/postgresql-binary.cabal
--- a/postgresql-binary.cabal
+++ b/postgresql-binary.cabal
@@ -1,7 +1,7 @@
 name:
   postgresql-binary
 version:
-  0.12.1.3
+  0.12.2
 synopsis:
   Encoders and decoders for the PostgreSQL's binary format
 description:
diff --git a/tasty/Main.hs b/tasty/Main.hs
--- a/tasty/Main.hs
+++ b/tasty/Main.hs
@@ -1,6 +1,6 @@
 module Main where
 
-import Main.Prelude hiding (assert, isRight, isLeft)
+import Main.Prelude hiding (assert, isRight, isLeft, select)
 import Control.Monad.IO.Class
 import Test.QuickCheck.Instances
 import Test.Tasty
@@ -101,14 +101,14 @@
                   let p = (,,) (PTI.oidPQ (PTI.ptiOID PTI.timestamptz))
                                ((A.encodingBytes . encoder) x) 
                                (LibPQ.Binary)
-                      x = read "2011-09-28 00:17:25"
+                      x = read "2011-09-28 00:17:25Z"
                   DB.unit "insert into a (b) values ($1)" [Just p]
                   DB.unit "set timezone to 'Europe/Stockholm'" []
                   textual <- DB.oneRow "SELECT * FROM a" [] LibPQ.Text
                   decoded <- fmap (B.valueParser decoder) (DB.oneRow "SELECT * FROM a" [] LibPQ.Binary)
                   return (textual, decoded)
               HUnit.assertEqual "" ("2011-09-28 02:17:25+02") textual
-              HUnit.assertEqual "" (Right (read "2011-09-28 00:17:25")) decoded
+              HUnit.assertEqual "" (Right (read "2011-09-28 00:17:25Z")) decoded
             ,
             timeRoundtrip "timestamptz" (fmap Apx Gens.auto) PTI.timestamptz
             ((. unApx) . bool A.timestamptz_float A.timestamptz_int)