Colin - couple of things I'd check.
First, the date on your PC. Just move the mouse over the date display at the bottom right of the screen and the full time/day/date should pop up. If it is wrong, you can right-click on the time display and select to adjust. Left click on that and you can re-do them to match reality.
For the time as shown on this bbs, go to My Home~Display Preferences and you will see a section that shows the time/date on the server and a block where you can adjust for local time if different.
Mine, for instance, is set at -5 since I am 5 hours earlier than the zone for this bbs.
If it isn't either of the items mentioned above, post details. Is your time correct and the day incorrect? Is it limited to this bbs or is everything on the PC affected such as the date/time stamp put on a new document when you create it?