If you are logged in while browsing, the forums go off the timezone that you have in your user profile. If you click the profile link, you will find a timezone dropdown. Central time is GMT - 6 hours.
Also, phpBB does not understand daylight saving time all that well. I had my profile set to GMT-4 (EDT) and now need to change it to GMT-5 (EST). Unfortunately, this action generally needs to be repeated at the beginning and end of every daylight saving time season. So during daylight saving time, you will need to change to GMT-5.
Hope that helps.