diff --git a/Network/HTTP/Date/Converter.hs b/Network/HTTP/Date/Converter.hs
--- a/Network/HTTP/Date/Converter.hs
+++ b/Network/HTTP/Date/Converter.hs
@@ -4,8 +4,6 @@
                                    , utcToHTTPDate
                                    ) where
 
-import Control.Applicative
-import Data.ByteString.Internal
 import Data.Time
 import Data.Time.Calendar.WeekDate
 import Data.Word
@@ -13,7 +11,7 @@
 import Foreign.Ptr
 import Foreign.Storable
 import Network.HTTP.Date.Types
-import System.IO.Unsafe (unsafePerformIO)
+import System.IO.Unsafe (unsafeDupablePerformIO, unsafePerformIO)
 import System.Posix.Types
 
 {-|
@@ -94,10 +92,10 @@
     isLeap year = year `rem` 4 == 0
               && (year `rem` 400 == 0 ||
                   year `rem` 100 /= 0)
-    (months, daysArr) = if isLeap yy
+    (mnths, daysArr) = if isLeap yy
       then (leapMonth, leapDayInMonth)
       else (normalMonth, normalDayInMonth)
-    findMonth n = inlinePerformIO $ (,) <$> (peekElemOff months n) <*> (peekElemOff daysArr n)
+    findMonth n = unsafeDupablePerformIO $ (,) <$> (peekElemOff mnths n) <*> (peekElemOff daysArr n)
 
 ----------------------------------------------------------------
 
diff --git a/http-date.cabal b/http-date.cabal
--- a/http-date.cabal
+++ b/http-date.cabal
@@ -1,5 +1,5 @@
 Name:                   http-date
-Version:                0.0.8
+Version:                0.0.9
 Author:                 Kazu Yamamoto <kazu@iij.ad.jp>
 Maintainer:             Kazu Yamamoto <kazu@iij.ad.jp>
 License:                BSD3
@@ -7,10 +7,11 @@
 Synopsis:               HTTP Date parser/formatter
 Description:            Fast parser and formatter for HTTP Date
 Category:               Network, Web
-Cabal-Version:          >= 1.8
+Cabal-Version:          >= 1.10
 Build-Type:             Simple
 
 Library
+  Default-Language:     Haskell2010
   GHC-Options:          -Wall
   Exposed-Modules:      Network.HTTP.Date
   Other-Modules:        Network.HTTP.Date.Converter
@@ -24,6 +25,7 @@
                       , time
 
 Test-Suite spec
+  Default-Language:     Haskell2010
   Type:                 exitcode-stdio-1.0
   HS-Source-Dirs:       test
   Main-Is:              Spec.hs
@@ -37,6 +39,7 @@
                       , time
 
 Test-Suite doctests
+  Default-Language:     Haskell2010
   Type:                 exitcode-stdio-1.0
   HS-Source-Dirs:       test
   Ghc-Options:          -threaded