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In most of Europe, when the clocks are set from daylight saving time to standard time, at 3 am the clocks are set back to 2 am. Thus, a time like 2.29 happens twice in that night. In spring, the clocks go from 1.59 to 3 am straight. Thus, 2.29 does not happen at all.

Imagine a plane leaving at 2.29 am from a certain place. When days it actually leave in both nights?

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I once had to spend I night in a train station in the UK (where I live) because there was no 2:30 train due to there being no such time as 2:30 that night. With a flight though, you'd be booked in advance, so one would hope this sort of problem wouldn't occur. –  Nathaniel Oct 30 '12 at 15:42
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I wrote a lot of the scheduling software for one of the major satellite providers in the mid 90's, and this was one of our worst headaches. A show would be scheduled for 2 am. Which 2 am? –  tcrosley Oct 30 '12 at 20:11
    
@tcrosley so what did you come up with in the end? –  bonifaz Oct 31 '12 at 12:40
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The scheduler GUI used to create the on-screen program guide had a bar-chart format (you could drag shows around, stretch or shrink them to adjust the duration etc.). So I ended up listing 2 am twice for the spring DST change, matching the day's 25 hours, and not showing it at all for the fall change, so the day had 23 hours. All the schedules were displayed in Eastern Time for the operator. However internally, they were kept in UTC time. I can't remember how the on-screen program handles the double 2 am case -- I'd check it out on Nov 4 but I live in Arizona where the time doesn't change. –  tcrosley Oct 31 '12 at 20:02
    
"In most of Europe, when the clocks are set from daylight saving time to standard time, at 3 am the clocks are set back to 2 am" - actually "The [EU] shifts all at once, at 01:00 UTC [i.e. GMT]". This could be 01:00, 02:00 or 03:00 local time. See en.wikipedia.org/wiki/Daylight_saving_time –  e100 Jan 14 '13 at 13:29
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3 Answers

up vote 10 down vote accepted

UTC is typically used for co-ordinating flight plans, air traffic control, and so on, for precisely this reason. UTC is 'standard' time in the sense that all other timezones, including daylight-adjusted ones, are set against it.

Of course, UTC is typically not used in day-to-day use by the public, so times in timetables, etc., will still typically be shown in local (daylight-adjusted time). In your examples, the only unambiguous way to express 2.29 when it occurs 'twice' would be to append the timezone identifier (for example, the UK uses GMT or BST, depending on the time of year). It isn't a problem very often!

I have flown from the US to Europe before during a mismatched week (the US adjusts away from DST a week before Europe). It wasn't a problem; the local times were just adjusted accordingly on the tickets, airline website, etc.

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A friend took an overnight boat from Sweden to Estonia when the clocks were put back, and didn't need to change his clock at all :) –  gerrit Oct 30 '12 at 19:58
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As for trains, in Sweden, trains at the end of DST will stop at the first station possible after 1:00 UTC (the instant the clocks are put back). As to the start of DST, all trains will instantaneously acquire a one hour delay at 1:00 UTC, and do their best to make up for the delay. If you take a night train at the start of DST, be prepared to have a one hour delay in the morning!

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I found an interesting discussion on Airliners.net, and some of the forum posters seem to be working in the industry. To summarize some of the comments, it can be a source of major confusion both for passengers, crew members (forgetting about DST switching and arriving late), ground personnel (longer shifts than usual) and apparently entire airlines, especially for flights between destinations that do not switch to DST at the same time (or one of it doesn't use DST at all).

For most parts of the world, it would seem that domestic flights wouldn't be affected much, as these are rare in such ungodly hour, however the situation is different for international flights. There is a blog rant on this very issue for US flights.

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