practice: Data.Time

test_time.hs

import System.Locale (defaultTimeLocale)
import Data.Time

-- 時刻をフォーマット
formatISO8601 :: ZonedTime -> String
formatISO8601 t = formatTime defaultTimeLocale "%FT%T%z" t

-- 時刻を表した文字列のパース (今回は Twitter の created_at)
parseCreatedAt :: String -> Maybe UTCTime
parseCreatedAt s = parseTime defaultTimeLocale "%a %b %d %H:%M:%S %z %Y" s

-- お試し
main :: IO ()
main = do
  -- formatting
  zt <- getZonedTime
  putStrLn $ formatISO8601 zt

  -- parsing
  let ct = "Sat Jun 02 07:31:54 +0000 2012" -- twitter の created_at
  case parseCreatedAt ct of
    Just t  -> do
      tz <- getCurrentTimeZone
      putStrLn . show $ utcToZonedTime tz t
    Nothing -> putStrLn "parse error"