I recently noticed that some of my servers configured to use GMT as their timezone (I will detail the merits of using GMT or UTC as a timezone in another post) had switched and where now reporting BST instead of GMT.
FreeBSD provides a utility tzsetup which provides a graphical user interface to select the time zone, unfortunately this list is sorted by geographic region, and is clumsy for what I want to do.
To set your system timezone manually, select the correct timezone entry from /usr/share/zoneinfo. When you find the timezone you want, copy it over to /etc/localtime. In my case:
# cp /usr/share/zoneinfo/Etc/GMT localtime
# adjkerntz -a (adjkerntz = adjust kernal timezone)