POSIX time, if you need to deal with timestamps and the like. Most people won't need this module.

You can use POSIXTime to obtain integer/word timestamps. For example:

import Data.Time
import Data.Time.Clock.POSIX
import Data.Int

nanosSinceEpoch :: UTCTime -> Int64
nanosSinceEpoch =
    floor . (1e9 *) . nominalDiffTimeToSeconds . utcTimeToPOSIXSeconds

main :: IO ()
main = do
    u <- getCurrentTime
    print $ nanosSinceEpoch u