I installed XFGuestbook today and noticed a time stamp problem too. My settings were working fine before and were set correctly in the the administration preferences. I tried to change the timezone offset in Extended Profile fields and that didn't work either.
The only thing that worked was changing my server timezone in the administration preferences. I messed around with it a couple of times and found the setting that brought the post in sink with the actual time. For example: It is correct at GMT -7:00 for the SERVER and GMT-5:00 for my default but I had to set the Server to GMT-2:00.
There must be something wrong in the code but I don't know what it is. Anyone know? It works fine for me now but I don't know if it will effect anything else and not in a good way. There has to be a more permanent and correct way to fix this problem.