type NominalDiffTime = DiffTime

[AshleyYakeley]

More-or-less nobody cares about leap-seconds, and those who do don't really need the extra safety of distinguishing NominalDiffTime and DiffTime.

Not sure what the impact of this will be, so not sure if we should do this.

[TravisWhitaker]

FWIW I currently have code that deals with deals with quantities where leap seconds are relevant, and where leap seconds are not. I'm not defending the current API per-se, but retaining the ability to distinguish between the different sorts of time intervals is a nice feature.