I suspect the time is correct but that you may have switched (locally) to daylight savings time. At present we do not support daylight savings only standard local times. Therefore during summer months (depending on whether your nation/state switches to daylight savings) the board times may appear to be one hour behind the real local time. The solution is to either put up with it (just let your users know) or to move the timezone of everyone who is affected (remember that not all your users may be from nations who have switched to daylight savings) on one hour, eg. GMT becomes GMT+1. This requires running some SQL and you should search our forums for information.
If this is not the case then I suspect your servers time is at fault. phpBB 2 stores all times in the form of a unix timestamp. This is a 32bit integer value containing the number of seconds since the unix epoch event, 1st January 1970 00:00:00 GMT. This value is global in nature and will be the same wherever you are in the world. So, if your time is incorrect and you have selected the appropriate timezone chances are your server time is off.
Also, im Admin-Bereich läßt sich das einstellen.
Im Übrigen kann der User die Zeitzone auch einstellen, wenn er eingelogt ist. Geht im PROFIL.