I recently moved from the Eastern time zone (GMT-5) to the Atlantic time zone (GMT-4). I've changed my time zone in My GTPlanet, but I'm still getting timestamps from an hour before :S
I think this is a bug within the "automatic DST" correction feature of the forum software. If you change that setting in your account options to "DST corrections always on", it should resolve the problem for you.
For whatever reason, it seems to be working fine now. I didn't change anything, just shut down my computer when I went out and then signed back in later on.