Behavior of negative OFFSET

I've been using a query on Postgres 8.4 with a negative OFFSET, which
works fine:

   SELECT DISTINCT s.* FROM s WHERE ... ORDER BY s.bday ASC, s.name
ASC LIMIT 15 OFFSET -15

When I run the same query on Postgres 9.1, I get an error:
   ERROR: OFFSET must not be negative


Question:
1. Was this changed in a version of Postgres? Is this configurable?
2. How do I get the original behavior of negative offsets?