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This might be a silly question but my thinking was the following:

There has to be a definition of time in an airplane for the logging of data and so on, but what definition would that be assuming a flight goes across timezones?

Would it be the local time at the airport of departure? Or at that of arrival? or maybe even a third, commonly agreed upon time (e.g., GMT)?

The idea came from a story of a friend that was traveling with the ship from Italy (GMT+1) to Greece (GMT+2). The cafeteria on board had a sign saying that it opens at 14:00, but on 14:20 local Italy time, while the ship was anchored in Italy, he was denied service by the crew saying that the sign was referring to Greece local time; where the ship "belongs".

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    $\begingroup$ Related, maybe a dupe? $\endgroup$
    – Pondlife
    Commented Jun 27, 2018 at 12:35
  • $\begingroup$ Certainly related. This question asks "what", that one asks "why". The question of dupe/no dupe could go either way. $\endgroup$
    – Steve V.
    Commented Jun 27, 2018 at 16:53
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    $\begingroup$ Some years ago I had the actual unfortunate (or fortunate depending on perspective) experience of a flight that had a stopover prior to arrival at my location. All THREE times were for 2:00 AM local - but the FUN part was it was on the day where daylight savings time changed so no-one knew the actual UTC time because of everything being listed in local times. They just gave me a 4 hour window as best they could do. $\endgroup$
    – Mark Schultheiss
    Commented Jun 27, 2018 at 21:21
  • $\begingroup$ There's a huge difference between what's used internally in the computers and what's displayed to customers - what are you asking about? $\endgroup$
    – pipe
    Commented Jun 28, 2018 at 8:24
  • $\begingroup$ @pipe Having been a customer myself, I know what is displayed and communicated. I am asking about the pilot-to-plane/ATC interactions, plane internals and so on. $\endgroup$
    – Ma0
    Commented Jun 28, 2018 at 9:10

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The pilot (and the ATC) use the UTC (Coordinated Universal Time). The flight's departure and arrival are in terms of the local times at the respective airports.

As for data logging, the FDR/CVR usually records in UTC (preferred) or the relative time count (usually count increments each 4 seconds of system operation). See Appendix M to Part 121 - Airplane Flight Recorder Specifications, for example.

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ICAO SARPs Annex 2 (Rules of the Air) states:

3.5 Time

3.5.1 Coordinated Universal Time (UTC) shall be used and shall be expressed in hours and minutes and, when required, seconds of the 24-hour day beginning at midnight.

3.5.2 A time check shall be obtained prior to operating a controlled flight and at such other times during the flight as may be necessary.

Note.— Such time check is normally obtained from an air traffic services unit unless other arrangements have been made by the operator or by the appropriate ATS authority.

3.5.3 Wherever time is utilized in the application of data link communications, it shall be accurate to within 1 second of UTC.

The above should apply to almost every country when it comes to international civil aviation. Militaries and uncontrolled flights are not covered by the above.

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The planes use UTC time. The same as ATC.

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    $\begingroup$ Thanks for the answer! A link to UTC would be great too (e.g., this) $\endgroup$
    – Ma0
    Commented Jun 27, 2018 at 12:20
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Let me add that pilots, ATC, and others use UTC, (not GMT) but it's common for customer-facing times to be in the time zone of the event. For example, a Flight from Florida to Alabama may land before it departs on the ticket.

Usually, ticket times are listed in local time. So Departing 1:00 pm Arriving 12:48 pm could actually appear on a ticket.

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  • $\begingroup$ Well, GMT might mean the same thing as UTC. It is, however, ambiguous. $\endgroup$
    – Jan Hudec
    Commented Jun 28, 2018 at 21:12
  • $\begingroup$ @coteyr, Please explain how pilots don't use GMT. If there is a difference between UTC, GMT, and Zulu I would like to understand it. Thanks. $\endgroup$
    – Michael Hall
    Commented Jan 20, 2020 at 23:24
  • $\begingroup$ timeanddate.com/time/gmt-utc-time.html its mostly semantic but UTC is a system of time where GMT is a time zone. $\endgroup$
    – coteyr
    Commented Jan 21, 2020 at 1:17
  • $\begingroup$ Zulu, on the other hand, is just shorthand for UTC + 0 Zero hours. For example, a time may be written 20:20z-5:00 (in theory) which is the same at 01:20z $\endgroup$
    – coteyr
    Commented Jan 21, 2020 at 1:21

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