No, the [interval](http://www.postgresql.org/docs/9.1/static/datatype-datetime.html) type [supports reduced precision](http://www.postgresql.org/docs/9.1/static/datatype-datetime.html) but none of the other date/time types do.

Postgres allows you to roll your own with [`create type`](http://www.postgresql.org/docs/9.1/static/sql-createtype.html) but unfortunately wont allow contraints to be added to the type which limits it's usefulness in this scenario.