Let us discuss time formats in Haskell. There are several different names and data types. First let me show you that every data type has his own IO action:

time:      getCurrentTime :: IO UTCTime
unix-time: getUnixTime    :: IO UnixTime
time       getPOSIXTime   :: IO POSIXTime
unix:      epochTime      :: IO EpochTime
time:      getZonedTime   :: IO ZonedTime
old-time:  getClockTime   :: IO ClockTime

I will not cover ClockTime, because it’s part of a deprecated package. Next we will start a ghci session to see what is the result of each function:

UnixTime:

λ: import Data.UnixTime 
λ: t <- getUnixTime >>= return . show . utSeconds
λ: read t :: Int
   1446987464

You can use toEpochTime and toClockTime.

POSIXTime:

λ: import Data.Time.Clock.POSIX
λ: t <- getPOSIXTime >>= return . show . round
λ: read t :: Int
   1446987600

Also, posixSecondsToUTCTime is part of the Library.

EpochTime:

λ: import System.Posix.Time
λ: t <- fmap show epochTime
λ: read t :: Int
   1446987733

λ: import Data.UnixTime 
λ: fmap fromEpochTime epochTime
   UnixTime {utSeconds = 1446988206, utMicroSeconds = 0}

UTCTime:

Last but not least let’s take a look at the time package:

λ: import Data.Time.Clock
λ: getCurrentTime >>= return . show

   No instance for (Show UTCTime) …

What is that? getCurrentTime has no visual representation aka can’t be converted to a string. No, we have to import another package:

λ: import Data.Time.Clock
λ: import Data.Time.Format
λ: t <- fmap show getCurrentTime
λ: read t :: UTCTime
   2015-11-08 13:28:23.095651 UTC

Well, that is a formated time string, but is it possible to have a simple timestamp like above. Sure:

 λ: import Data.Time.Clock
 λ: import Data.Time.Format
 λ: t <- getCurrentTime >>= return . formatTime defaultTimeLocale "%s"
 λ: read t :: Int
    1446989906

ZonedTime:

All times we’ve got are in UTC. The main advantage of the time package is it can handle timezones, too:

λ: import Data.Time.LocalTime 
λ: import Data.Time.Format 
λ: getZonedTime >>= return . formatTime defaultTimeLocale "%Y-%m-%d, %H:%M %Z"
   "2015-11-08, 15:00 CET"

At last there is one question. How to convert a timestamp back to a formated string:

λ: import Data.Time.Clock
λ: import Data.Time.Clock.POSIX 
λ: import Data.Time.LocalTime 
λ: import Data.Time.Format 
λ: let s = read "1446991464" :: Int
λ: let t x = (posixSecondsToUTCTime . realToFrac) x
λ: utcToLocalZonedTime (t s) >>=
     return . formatTime defaultTimeLocale "%Y-%m-%d, %H:%M %Z"
   "2015-11-08, 15:04 CET"