The forum doesn't use client-side generated timestamps. All the timestamps are generated by the server and are in GMD by definition.
There are three chances for troubles, I see:
1. Incorrect TZ at the server (i.e. gmdate() gives incorrect result).
2. Incorrect TZ at the client-side: user just has incorrect time locally set.
3. Incorrect browser behavior: cookie is removed too early. Maybe browser has modified the expiration date for the cookie (it is sent in GMT from the server and must be untouched).
Please, check these cases and report if your trouble doesn't match them.