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E.g., in the US, PT time zone, on Sunday, November 6, 2022, 2:00:00 am PT, the clock was reset to Sunday, November 6, 2022, 1:00:00 am PT. If I book a plane scheduled to depart on November 6, 2022, 1:30:00 am PT, how can I know whether the plane is scheduled to depart before the clock is reset?

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    The question would be more interesting if completely rewritten and generalized among different means of transportation: trains, busses and companies like Amtrak (hold the trains for 1 hour at the nearest station during the change) or others. Commented Dec 8, 2022 at 1:30
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    For what it's worth, German trains get cancelled if they are scheduled to start during a non-existing time (in March) or an ambiguous time (in October). This doesn't affect many trains, though. eurogunzel.com/2019/04/… Commented Dec 8, 2022 at 7:08
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    @EricDuminil Then again, German trains get cancelled all the time for whatever reasons ... Commented Dec 8, 2022 at 12:42
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    That makes sense - time is ambiguous such that there would be a train with the same time before and after changing the clock (still 1 hr apart)? Better cancel both. Deutsche Bahn ...
    – DonQuiKong
    Commented Dec 8, 2022 at 13:15
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    @EricDuminil, If it's a long-distance train from say Paris to Warsaw, hopefully they don't cancel the whole train just because of one or two stops along the way that fall in the time change-over.
    – The Photon
    Commented Dec 9, 2022 at 22:12

4 Answers 4

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You can usually tell from the flight duration.

Timetables and listings will show a flight duration. If one of the times, departure or arrival, is unambiguous, you can compute the other time by adding or subtracting the flight duration.

This may not work for flights less than an hour within the same timezone, or if the flight direction/speed perfectly follows the DST changeover, but will handle the rest of the cases.

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    You can also compare the duration to the previous and following week. Commented Dec 7, 2022 at 22:00
  • Now we need to worry about what if it's a flight from LA to San Diego and it's less than an hour long and both departure and arrival fall in the ambiguous time period.
    – The Photon
    Commented Dec 9, 2022 at 22:08
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The clock resets on November 6. 2022, 2:00:00 am PDT to 1:00:00 am PST.

You should check the schedule. If it shows 1:30:00am PDT - then it is before the reset. If it shows 1:30:00am PST - then it is after the reset.

In the US there are no areas in the PT timezone that do not observe daylight savings, so this distinction should appear in all the schedules. If it doesn't - you'll need to contact whoever publishes the schedule for clarifications.

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    What if the schedule doesn't explicitly indicate the time zone? Often they just say something to the effect of "All times are local time." Or what if they indicate just PT without specifying standard or daylight? Are you saying that even if the schedule doesn't usually indicate the time zone, they will in this case? Commented Dec 7, 2022 at 16:29
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There will be very few scheduled flights at that time of the day (or any other scheduled activities, which is the reason the switch is made in the middle of the night); however it happens.
The flight will explicitly specify which time is meant; before or after the switch. I have seen that before, but have no details to show prove.

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    I would be curious to see how this can be shown and transmitted across the wide variety of systems and devices. Maybe a few systems will show it, but I wouldn't count on it universally.
    – jcaron
    Commented Dec 7, 2022 at 21:39
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    @jcaron As with most aviation and computer systems, the standard is to use UTC time, the conversion to local time happens at the user display stage. (E.g. AIDX XML Implementation Guide "Are flight times specified in local time or Zulu (UTC) time? Times are always specified in UTC in AIDX, and a “Z” appended to the time value, e.g. '2012-04-13T13:32:50Z'")
    – user71659
    Commented Dec 7, 2022 at 21:43
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    @user71659: Of course, people would have to actually use AIDX XML. Working at Amadeus, I can say with confidence that most transmissions in TTY or EDIFACT just used "a" time, with no timezone associated, and it was implicit this was the local time at the given airport. And yes, this didn't work so well with DST... Commented Dec 8, 2022 at 13:02
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    @user71659 It would indeed be the right way to do it, but given the number of legacy systems out there, I would have been very surprised if all were indeed using UTC. While on the "flying" side (flight planning etc.) UTC is quite generalised, in booking systems it's another matter, as Matthieu M. confirms.
    – jcaron
    Commented Dec 8, 2022 at 13:04
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    The traditional internal record for flights is text records in "GDS" and "CRS" systems such as "PNRs". If you look at examples of that format you'll see flight times specified in 4 digits representing "local time"; an arrival time of "0130+1" means "1:30 AM 1 day after departure". As shown in my previous comment, even recently designed APIs like "NDC" retain this convention of using "local time". Even if a system shows a timezone, I would be suspicious that it has guessed it by location, and the source of truth did not actually indicate it.
    – IMSoP
    Commented Dec 9, 2022 at 17:35
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I have experienced this often, flying late in the day following the "spring forward" change. Every single time, my 2100 hrs departing flight has been "delayed" by one hour.

As the planes fly multi-leg schedules (of non-stop flights), it really doesn't matter what the ticket says, the flight can't miraculously make up the hour on the time change. It's going to depart at 0500 UTC until there's a sufficient ground layover to catch up. The airlines will not post a custom schedule while the hour gets ironed out across the system; customers will expect their 2100 hrs flight departs at that time "all the time". It's possible of course, the airline already made that up earlier in the route, so if the prior leg had favourable headwinds, maybe it got made up already. So one time, it was only delayed 35 minutes.

I have even contacted the airline the day of the flight to ask the flight status. They assured me it's on time, until the display showed it wasn't.

The TL;DR is your 1:30:00 am PT flight will depart at 1:30:00 am PT. Most likely that's before the timeshift, though your flight may be "delayed an hour, until it's 1:30:00 am PT again!

You may have more luck getting precise information from the airline if you have connecting flights, but now it's a multi-factor problem!

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