diff --git a/cbits/conv.c b/cbits/conv.c
--- a/cbits/conv.c
+++ b/cbits/conv.c
@@ -1,17 +1,18 @@
 #include "config.h"
 
-#include <time.h>
-#include <locale.h>
-
 #if IS_LINUX
 /* Linux cheats AC_CHECK_FUNCS(strptime_l), sigh. */
 #define THREAD_SAFE 0
 #define _XOPEN_SOURCE
+#define _BSD_SOURCE
 #elif HAVE_STRPTIME_L
 #define THREAD_SAFE 1
 #else
 #define THREAD_SAFE 0
 #endif
+
+#include <time.h>
+#include <locale.h>
 
 #if THREAD_SAFE
 #include <xlocale.h>
diff --git a/test/Test.hs b/test/Test.hs
new file mode 100644
--- /dev/null
+++ b/test/Test.hs
@@ -0,0 +1,103 @@
+{-# LANGUAGE OverloadedStrings, TemplateHaskell #-}
+
+module Main where
+
+import qualified Data.ByteString.Char8 as BS
+import Data.UnixTime
+import System.Locale
+import System.Time hiding (toClockTime)
+import Test.Framework.Providers.HUnit
+import Test.Framework.TH.Prime
+import Test.Hspec.Expectations
+
+----------------------------------------------------------------
+
+main :: IO ()
+main = $(defaultMainGenerator)
+
+----------------------------------------------------------------
+
+case_formatUnixTime :: Expectation
+case_formatUnixTime = res `shouldBe` ans
+  where
+    res = formatUnixTime mailDateFormat $ UnixTime 0 0
+    ans = "Thu, 01 Jan 1970 09:00:00 +0900"
+
+case_formatUnixTimeGMT :: Expectation
+case_formatUnixTimeGMT = res `shouldBe` ans
+  where
+    res = formatUnixTimeGMT webDateFormat $ UnixTime 0 0
+    ans = "Thu, 01 Jan 1970 00:00:00 GMT"
+
+case_parseUnixTime :: Expectation
+case_parseUnixTime = res `shouldBe` ans
+  where
+    res = parseUnixTime mailDateFormat "Thu, 01 Jan 1970 09:00:00 +0900"
+    ans = UnixTime 0 0
+
+case_parseUnixTime2 :: Expectation
+case_parseUnixTime2 = res `shouldBe` ans
+  where
+    res = parseUnixTimeGMT webDateFormat "Thu, 01 Jan 1970 00:00:00 GMT"
+    ans = UnixTime 0 0
+
+case_formatParse :: Expectation
+case_formatParse = do
+    ut@(UnixTime sec _) <- getUnixTime
+    let ut' = parseUnixTime mailDateFormat $ formatUnixTime mailDateFormat ut
+    ut' `shouldBe` UnixTime sec 0
+
+----------------------------------------------------------------
+
+case_fromClockTime :: Expectation
+case_fromClockTime = do
+    ct <- getClockTime
+    let fmt1 = formatUnixTime "%a, %d %b %Y %H:%M:%S" $ fromClockTime ct
+    cal <- toCalendarTime ct
+    let fmt2 = BS.pack $ formatCalendarTime defaultTimeLocale "%a, %d %b %Y %H:%M:%S" cal
+    fmt1 `shouldBe` fmt2
+
+case_toClockTime :: Expectation
+case_toClockTime = do
+    ut <- getUnixTime
+    let fmt1 = formatUnixTime "%a, %d %b %Y %H:%M:%S" ut
+        ct = toClockTime ut
+    cal <- toCalendarTime ct
+    let fmt2 = BS.pack $ formatCalendarTime defaultTimeLocale "%a, %d %b %Y %H:%M:%S" cal
+    fmt1 `shouldBe` fmt2
+
+----------------------------------------------------------------
+
+case_diffTime :: Expectation
+case_diffTime = do
+    ut0 <- getUnixTime
+    ut1 <- getUnixTime
+    let ut0' = addUnixDiffTime ut1 $ diffUnixTime ut0 ut1
+        ut1' = addUnixDiffTime ut0 $ diffUnixTime ut1 ut0
+    ut0' `shouldBe` ut0
+    ut1' `shouldBe` ut1
+
+----------------------------------------------------------------
+
+case_diffTimeFromSeconds :: Expectation
+case_diffTimeFromSeconds = do
+    res1 `shouldBe` ans
+    res2 `shouldBe` ans
+    res3 `shouldBe` ans
+  where
+    base = parseUnixTime mailDateFormat "Tue, 22 Nov 2011 06:49:58 +0900"
+    ans = parseUnixTime mailDateFormat "Tue, 22 Nov 2011 06:50:02 +0900"
+    res1 = addUnixDiffTime base 4
+    res2 = addUnixDiffTime base (secondsToUnixDiffTime (4 :: Int))
+    res3 = addUnixDiffTime base (microSecondsToUnixDiffTime (4000000 :: Int))
+
+----------------------------------------------------------------
+
+case_diffTimeToSeconds :: Expectation
+case_diffTimeToSeconds = res `shouldBe` ans
+  where
+    ans :: Rational
+    ans = -12.345678
+    res = realToFrac $ microSecondsToUnixDiffTime (-12345678 :: Int)
+
+----------------------------------------------------------------
diff --git a/unix-time.cabal b/unix-time.cabal
--- a/unix-time.cabal
+++ b/unix-time.cabal
@@ -1,5 +1,5 @@
 Name:                   unix-time
-Version:                0.1.1
+Version:                0.1.2
 Author:                 Kazu Yamamoto <kazu@iij.ad.jp>
 Maintainer:             Kazu Yamamoto <kazu@iij.ad.jp>
 License:                BSD3
@@ -7,12 +7,13 @@
 Synopsis:               Unix time parser/formatter and utilities
 Description:            Fast parser\/formatter\/utilities for Unix time
 Category:               Data
-Cabal-Version:          >= 1.6
+Cabal-Version:          >= 1.10
 Build-Type:             Configure
 Extra-Source-Files:     cbits/conv.c cbits/config.h.in configure configure.ac
 Extra-Tmp-Files:        config.log config.status autom4te.cache cbits/config.h
 
-library
+Library
+  Default-Language:     Haskell2010
   GHC-Options:          -Wall
   Exposed-Modules:      Data.UnixTime
   Other-Modules:        Data.UnixTime.Conv
@@ -23,6 +24,21 @@
                       , bytestring
                       , old-time
   C-Sources:            cbits/conv.c
+
+Test-Suite spec
+  Type:                 exitcode-stdio-1.0
+  Default-Language:     Haskell2010
+  Hs-Source-Dirs:       test
+  Main-Is:              Test.hs
+  GHC-Options:          -Wall
+  Build-Depends:        base
+                      , bytestring
+                      , hspec-expectations
+                      , old-locale
+                      , old-time
+                      , test-framework-hunit
+                      , test-framework-th-prime
+                      , unix-time
 
 Source-Repository head
   Type:                 git