If you need to check your server time, you need to check that the time actually corresponds with the time zone. For example, this post is at ~12:50 PM in the UK, but that of course is GMT+1 right now. If it's 12:50 on the server but in a time zone that is GMT (for example), that will make times incorrect.
I noticed it last night whilst online at just after 11pm GMT, posts I had just made were showing as being posted Yesterday, then when I received an email to say I had a new conversation the email time was shown as an hour out too?
Just double checked my server time through WHM (root); "Current Time Tue Jun 21 13:04:29 GMT 2011" so that's correct...
I'm not familiar with what the "GB" time zone means. Europe/London is likely what you want.
That said, it doesn't really matter if the time zone you select follows DST, as the server time itself is still about an hour out. (I should note that you should be careful when changing it; if you're moving the time back by an hour, some things will get confused until the time catches back up to what it was.)