This will be a consequence of the board having been run on 2 different servers with different time zones.
The posts are stored with the actual server time on it. On the old server, the time was GMT, so were correct for us.
When we moved to the new one, it's US time (not sure which), and is x hours out.
To get around this, the forum allows you to add/subtract a fixed amount of time from the stored post to be displayed. Now we have the correct time being shown for the US servers, but not the old posts.