I took a quick look at phpbb.com and searched for this topic, but I didn't find any clues. Their bug report list is HUGE though, and I could have easily missed it, so it still might be a known bug though :-)
The bug sounds strange. Tried to log out of the forum and in again? What time settings do you have in your profile? Have you had this problem all the time, or did it occur lately?
eivindm