The only place I can find in AR that doesn't treat `:datetime` and `:timestamp` identically is here: https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/postgresql/quoting.rb#L53

IMO, the spellings of timestamp that don't match that regexp were merely an oversight in the previous implementation, and the PG adapter (at least) should use one value consistently. `:datetime` seems to be the more prevalent.