# How are flight schedules displayed without ambiguity on DST change days?

Flight schedules are based on the local time of the station of departure or arrival. Twice a year though, this is either ambiguous or impossible. What happens on these boundaries?

Say, for instance, a flight is scheduled for departure daily at 1:30 A.M. This year, DST ends at November 2nd, 2 A.M., meaning the local clock will go from 2 A.M. back to 1 A.M. Will this flight depart on the first 1:30 A.M. or the second?

And the other way round, say next year a flight is scheduled for departure daily at 2:30 A.M. When DST starts, the clock will go directly from 2:00 A.M. to 3:00 A.M. and 2:30 doesn't exist. What happens for this flight?

• Flight schedules are based on UTC, at least in all ATC / ATM systems. – DeltaLima Oct 24 '14 at 13:22
• Flight schedules are definitely not based on UTC; they are based on local time. The answer to this (good) question isn't really about aviation, but about timetabling. The answer is that the timetable will have a footnote noting and clarifying the ambiguity on that particular day. – Hugh Oct 26 '14 at 2:50
• @Hugh At the the four airlines I worked for, the flight schedules as maintained by dispatch were all in UTC. It was the responsibility of the individual stations and marketing to translate them to local for public dissemination. – Terry Feb 25 '15 at 4:00
• For what it's worth, in 10 years of international flying I never had any trouble caused by DST transitions in spite of the fact that there's a great variety in the datse of implementation between countries. If one was curious what the transition dates were for a given country, there were pages in our Jepp manuals that gave that information. – Terry Feb 25 '15 at 4:10

It's only really ambiguous once a year, when moving from summer time to winter time and the hour is 'repeated' - in this case, departures and arrivals are scheduled in the 'first' instance of the hour: eg if the flight is scheduled 01:30, it will depart 1.5 hours after midnight. The clock will then go back eg at 2am, and the airport will be relatively for one hour with no departures (although, of course, arrivals may still come in)

On the other shift, the hour is 'lost' so the departure takes place later. In practice, no planes are scheduled to arrive or depart in this missing hour, and since almost all nations make the change in the middle if the night when airspace is very quiet and most airports have no departures at all there is virtually no knock-on effect. Arrivals arrive when they arrive, departures are simply scheduled for a 'real' hour, if there are even departures at all at that time of day anyway.

Note that this is only for passenger scheduling - in air traffic control etc all times are Zulu (UTC) which has no ambiguity.

TL;DR: Fall/Autumn, aircraft depart in the "first" instance of the hour. Spring: scheduling works as usual, but with no flights scheduled in the "missing" hour (eg 2-3am)

• What does "scheduled in the 'first' instance of the hour" mean? For example if 2:30 UTC and 3:30 UTC both map to the same local time, are you saying that nothing is ever scheduled for 3:30 UTC? Do you have any citation, any evidence for saying that? – ChrisW Oct 24 '14 at 22:18
• Okay, perhaps 'never' is a stretch, as there may be cases where the aircraft has to hit a landing slot elsewhere. Perhaps it's more accurate to say 'rarely' and note that the airline would almost always issue communicate this to the customer and travel agent (eg via an SSR on the PNR) – Jon Story Oct 24 '14 at 22:21
• @JonStory In practice, no planes are scheduled to arrive or depart in this missing hour Can you please provide reference for this statement? – Farhan Oct 25 '14 at 0:42
• My little experience is that planes tend not to be scheduled for the early hours of the morning. E.g. for trans-Atlantic flights, Europe-to-America (east coast) leaves in the morning so that it arrives in the afternoon or evening, whereas America-to-Europe leaves in the late afternoon or evening so that it arrives in the late morning or early afternoon the next day. Airports are mostly closed at 3 in the morning. – ChrisW Oct 25 '14 at 10:29
• As Chris said, there are very few departures in the early hours anyway. The majority of those that do exist are scheduled to leave in the 'first' hour, before the clocks change. The only time I've seen otherwise is to hit an arrival slot. – Jon Story Oct 25 '14 at 10:31

Daylight Savings does not cause ambiguity in flight schedules, even though it may appear to.

## Basic Concepts

Before discussing your specific scenarios, let's look at some concepts.

• First of all, not all countries observe DST. The ones which do, may not observe it for the same period either. Hence, airlines know if and when the destination region1 observes DST.

• A flight's duration does not change because of DST. If an airplane takes 5 hours to reach its destination, it won't take 4 or 6 hours because DST is turned on or off.

• Airlines and related entities (e.g. ATC) use times UTC, especially when an airplane is crossing a timezone, to reduce confusion. Pilots and ATC personal are used to follow UTC times without any difficulty. UTC does not observe DST.

• When a timezone observes DST, they change the timezone code too. For example, instead of EST (Eastern Standard Time), they'll use EDT (Eastern Daylight Time).

• When DST ends, clocks are moved from 3AM to 2AM, not from 2AM to 1AM.

Now, let us talk about the scenarios you mentioned.

1. If a flight is departing at 2:30AM (time corrected as I mentioned above), it will depart most probably on the first 2:30AM. Flights have their schedules mentioned in UTC as well, so it will not a confusion. As I do not know everything, I have never seen a flight departure time in that particular hour. Would you care to share an example?

2. If a flight is arriving at supposedly 2:30AM on the day when DST is starting, it will not arrive at 2:30AM but at 3:30AM. Because, as you said, 2:30AM does not exist on that day for that timezone.

## Examples

As suggested by fooot, I'm adding an example. This flight is from JFK to AUH.

This first example is on November 1st (when DST is in effect).

Departure    Arrival
EDT 12:40    EDT 01:10
UTC 16:40    UTC 05:10
GST 20:40    GST 09:10 (GST: Gulf Standard Time)

Flight Duration: 12:30


This is the same flight on November 2nd (when DST is not in effect).

Departure    Arrival
ET  11:40    ET  00:10
UTC 16:40    UTC 05:10
GST 20:40    GST 09:10 (GST: Gulf Standard Time)

Flight Duration: 12:30


Footnotes

1: Not all parts of a single country may observe DST. In USA (for example), most parts of Arizona do not observe DST. I agree with them, DST does not make any sense or provide any benefit in today's world.

• relevant extra information on DST: youtube.com/watch?v=84aWtseb2-4 (especially the segment at 5:10 for Arizona) – Federico Oct 24 '14 at 14:48
• I think it would be helpful to show an example, with the departure/arrival times in Standard/Daylight/UTC – fooot Oct 24 '14 at 14:53
• Contrary to your first sentence, it is ambiguous at all: in the case where you say, "it will depart most probably on the first 2:30AM", what if it doesn't and departs instead on the second 2:30AM? Also, 3AM to 2AM, or from 2AM to 1AM, depends on (varies with) the local jurisdiction. – ChrisW Oct 24 '14 at 15:46
• IIRC: EST = Eastern Standard Time and EDT = Eastern Daylight (savings) Time – CGCampbell Oct 24 '14 at 19:01
• In UK the switch is from 2am to 1am, because Europe switches at once between 2:00 CET and 3:00 CEST, i.e. at 1:00 UTC. – Jan Hudec Oct 25 '14 at 9:53