Hi Phil,
Simple-date, and the glue that integrates it in Postmodern, is no longer loaded by default. Load the :simple-date-postgres-glue system to get the old behaviour.
I actually recommend people to use local-time rather than simple-date now (it also has cl-postgres glue code in its distribution now), but if you don't care about time zones, and don't want to update your code. simple-date still works.
Best, Marijn