To set the timezone on Debian, type tzselect at a terminal prompt, and answer the questions. For the UK, the answers are 8 (Europe), 8 (Britain) and 1 (yes).
The only annoying thing is that you have to put it into your profile and the easiest way to do this is to type the following:
echo "TZ='Europe/London'; export TZ" >.profile
Note the mixture of quotes.
Perhaps nobody cares about the time on their computer, but I do!
[moderator intervention] As pointed out here viewtopic.php?f=91&t=4977#p1225922 since two years ago, (its now 2019) there is a simple option in the raspi-config utility to accomplish changing the timezone information needed by the Raspbian Operating System.
This thread is now only of historical importance.