diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,29 @@
+Copyright (c) 2012, IIJ Innovation Institute Inc.
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions
+are met:
+
+  * Redistributions of source code must retain the above copyright
+    notice, this list of conditions and the following disclaimer.
+  * Redistributions in binary form must reproduce the above copyright
+    notice, this list of conditions and the following disclaimer in
+    the documentation and/or other materials provided with the
+    distribution.
+  * Neither the name of the copyright holders nor the names of its
+    contributors may be used to endorse or promote products derived
+    from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
diff --git a/Setup.hs b/Setup.hs
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--- /dev/null
+++ b/Setup.hs
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+import Distribution.Simple
+main = defaultMain
diff --git a/System/Date/Cache.hs b/System/Date/Cache.hs
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--- /dev/null
+++ b/System/Date/Cache.hs
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+-- |
+-- Formatting time is slow.
+-- This package provides mechanisms to cache formatted date.
+module System.Date.Cache (
+  -- * Types
+    DateCacheConf(..)
+  , DateCacheGetter
+  , DateCacheCloser
+  -- * Date cacher
+  , ondemandDateCacher
+  , clockDateCacher
+  ) where
+
+import Control.Applicative
+import Control.Concurrent
+import Data.ByteString (ByteString)
+import Data.IORef
+
+type DateCacheGetter = IO ByteString
+type DateCacheCloser = IO ()
+
+data DateCache t = DateCache {
+    timeKey :: !t
+  , formattedDate :: !ByteString
+  } deriving (Eq, Show)
+
+data DateCacheConf t = DateCacheConf {
+    -- | A function to get a time. E.g 'epochTime' and 'getCurrentTime'.
+    getTime :: IO t
+    -- | A function to format a time.
+  , formatDate :: t -> IO ByteString
+  }
+
+newDate :: DateCacheConf t -> t -> IO (DateCache t)
+newDate setting tm = DateCache tm <$> formatDate setting tm
+
+-- |
+-- Date cacher which gets a time and formatted it only when
+-- returned getter is executed.
+ondemandDateCacher :: Eq t => DateCacheConf t -> IO (DateCacheGetter, DateCacheCloser)
+ondemandDateCacher setting = do
+    ref <- getTime setting >>= newDate setting >>= newIORef
+    return $! (getter ref, closer)
+  where
+    getter ref = do
+        newTm <- getTime setting
+        cache <- readIORef ref
+        let oldTm = timeKey cache
+        if oldTm == newTm then
+            return $ formattedDate cache
+          else do
+            newCache <- newDate setting newTm
+            writeIORef ref newCache
+            return $ formattedDate newCache
+    closer = return ()
+
+-- |
+-- Date cacher which gets a time and formatted it every second.
+-- This returns a getter.
+clockDateCacher :: Eq t => DateCacheConf t -> IO (DateCacheGetter, DateCacheCloser)
+clockDateCacher setting = do
+    ref <- getTime setting >>= newDate setting >>= newIORef
+    tid <- forkIO $ clock ref
+    return $! (getter ref, closer tid)
+  where
+    getter ref = formattedDate <$> readIORef ref
+    clock ref = do
+        threadDelay 1000000
+        tm <- getTime setting
+        date <- formatDate setting tm
+        let new = DateCache {
+                timeKey = tm
+              , formattedDate = date
+              }
+        writeIORef ref new
+        clock ref
+    closer tid = killThread tid
diff --git a/date-cache.cabal b/date-cache.cabal
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--- /dev/null
+++ b/date-cache.cabal
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+Name:                   date-cache
+Version:                0.3.0
+Author:                 Kazu Yamamoto <kazu@iij.ad.jp>
+Maintainer:             Kazu Yamamoto <kazu@iij.ad.jp>
+License:                BSD3
+License-File:           LICENSE
+Synopsis:               Date cacher
+Description:            Formatting time is slow. This package provides
+                        mechanisms to cache formatted date
+Category:               System
+Cabal-Version:          >= 1.8
+Build-Type:             Simple
+
+Library
+  GHC-Options:          -Wall
+  Exposed-Modules:      System.Date.Cache
+  Build-Depends:        base >= 4 && < 5
+                      , bytestring
+
+Source-Repository head
+  Type:                 git
+  Location:             git://github.com/kazu-yamamoto/logger.git