How to format UTCTime as ISO 8601 in Haskell

I don't see any pre-existing function for doing this, but you can make one easily using Data.Time.Format.formatTime:

import System.Locale (defaultTimeLocale)
import Data.Time.Format (formatTime)

iso8601 :: UTCTime -> String
iso8601 = formatTime defaultTimeLocale "%FT%T%QZ"

(You need to convert the time to a UTCTime before passing it to this function in order for it to actually display the actual UTC time.)

The time library in version 1.9 added this functionality (docs):

iso8601Show :: ISO8601 t => t -> String

and UTCTime has a ISO8601 instance.