GMT. For about the tenth year, I am leaving my clocks well alone this weekend. There are defensible arguments for doing regular activities a bit earlier in summer, but changing the clocks twice a year just isn't a sensible way of doing it. From an information systems point of view, it's just bad data modelling. It would be much easier if we could assume that every day has 24 hours (give or take a second), that every day has a 1:30 am (and no day has two), and that clocks will run by themselves.
Most timetables get revised regularly anyway -- train timetables, for example, usually change around the time of the clock shifts. Both at school and at one past job, we had summer timetables and winter timetables without causing any problems.
I don't know what the average number of clocks per person is these days, but I don't think it's unusual to have one in every room, plus watches, cars, microwave ovens etc. etc. BST probably costs about 30-60 minutes of leisure time per person per year. Add about one programmer-day per medium sized business unit per year dealing with errors thrown up by the time shift. Add the cost of all the human errors introduced twice every year. Add the costs of those that need their timetables to stay regular, and therefore have to adjust them to compensate. (I wouldn't like to be running an airline at this time of year). Compare to the benefits for people who would otherwise have to change their schedules to take advantage of the early mornings, but this way don't.
If I were to pluck a figure out of the air, I'd say this insanity costs the country about a billion pounds a year. It's going to get worse, not better. I don't know how many people routinely have to know the time in more than one timezone, but it makes that much more difficult. Even if we bully the whole world to do the shift at the same date, half the world is still doing it the other way.
My refusal to change my clocks is a token resistance. What's most frustrating is that most people agree it's silly to do this, but nobody cares much. I try to keep the issue visible, to remind people what they're losing.
All of the arguments in defense of the nonsense assume that people are totally incapable of noticing or adapting to the change of the seasons without the government tricking them into it. I have a higher opinion of my countrymen than that.
Update: via Stumbling and Mumbling, a study estimating the effect of clock changes on the equities market. I'm not really sure what to make of that -- the author's explanations are in terms of subjective effects on traders rather than effects on the actual companies being traded, so I wouldn't count the devaluation detected as a "loss" per se. The author also mentions in passing increased accident rates following clock changes, which I hadn't taken into consideration.
Speaking of subjective effects, I think that my habit of adjusting my timetable rather than my clocks helps me to adjust as regards sleep and so on. There's no logical reason for that, but who knows what effect our belief as to what time it is affects our minds?
Also cited, an earlier item from Village Hampden. I'd meant to refer to that, but I couldn't find it today. I see BST more as a symbol of government's inflated self-importance than its tyranny -- after all, legally everyone could do as I do and leave their clocks alone. It is worthwhile to demonstrate that the state doesn't control the stars and the planets...
Update 2: Via the Risks Digest (in full, the Association for Computing Machinery Forum on Risks to the Public in Computers and Related Systems), Barclays' cash machine network brought down for a day by the time change. Every spring and autumn brings similar news stories.