Whenever I use formatTime it's always with defaultTimeLocale. Why not make TimeLocale an instance of Def from Data.Default? http://hackage.haskell.org/package/data-default where def = defaultTimeLocale.

This would mean having to add data-default to GHC's distribution, but it's rather small and useful. The alternative is adding the instance in the data-default package, but then data-default depends on time-locale and we're still encouraging use of defaultTimeLocale. Swings and roundabouts.

Just an idea.