I guess you are using simple-date. If simple-date gets loaded after cl-postgres, it registers itself as the preferred date format. If it gets loaded before, nothing happens. That probably explains the strangeness. There is an extra system, :simple-date-postgres-glue, defined in simple-date.asd, which can be loaded to ensure the glue code is used.
Does clbuild pull from the repository, or load the latest released version?