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Edit: this is a very silly question, sorry about that :/ I won't delete it though.

When I was little and learned about the IDL, my first thought was the following: if I could do a couple of circles around the North Pole on a plane, then I would have to add several days to the calendar, and therefore I would be able to skip school days! 😊

While this is obviously not true, this means that the line stops making any sense directly near the poles, where one can cross it even by simply walking around the pole. I have stumbled upon a video about the IDL and suddenly remembered my childhood dreams of skipping school this way, and realized that I still don't quite understand why exactly does the concept of IDL break down.

The problem is not latitude: if I go around the equator at 0.99c, I will cross the IDL 7 full times in 1 second, yet just one second would have passed (obviously ignoring relativistic effects), and consequently, there will be no need to adjust my calendar. Perhaps the problem is the angular velocity around the Earth's rotation axis? But if for slow speeds we adjust the day, and for fast ones we don't, then what happens in the middle? And how do polar expeditions keep track of time then?

I'd be glad to see an explanation for all that.

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    $\begingroup$ Keep in mind, you don't only change times when crossing the dateline! For example, if you head west, yes, you gain a day crossing the IDL. But then as you continue west... you lose hours, one at a time (generally), as you cross into each new timezone. And so if you circle the world, you'll be right back at the same time you started at (other than the time elapsed), having lost 24 hours in the end after having gained the day. $\endgroup$ Nov 8, 2023 at 8:32
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    $\begingroup$ @JeopardyTempest I can't believe this idea hasn't crossed my mind even once 🤯 This + Antartica partitioning pretty much answers my question. $\endgroup$
    – DL33
    Nov 14, 2023 at 15:49

3 Answers 3

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Time zones and the date line are associated with the geography, not the traveler. In practical and mechanical terms, you don't "gain" or "lose" days by crossing the date line repeatedly; you're just adjusting to the local time and date.

In the Antarctic, for the sake of convenience, the various research stations use whatever time zone is most useful for them for interacting with their home countries or other nations of interest. International (and scientific data) coordination generally standardizes on Coordinated Universal Time (UTC), formerly known as Greenwich Mean Time (GMT). In the Arctic, of course, there's no land at or near the pole.

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  • $\begingroup$ Yes, obviously, I'm only adjusting to local time, but the rule still is that I have to add or subtract a day for the local time. I clearly wouldn't need to do that if I traveled around the planet in one second, so the rule doesn't always hold. Why? $\endgroup$
    – DL33
    Nov 6, 2023 at 19:08
  • $\begingroup$ The insights about the Antarctica make sense. I guess that near the poles the concepts of "day" and "night" are so broken that the 24-hour clocks don't make much sense anyway. $\endgroup$
    – DL33
    Nov 6, 2023 at 19:11
  • $\begingroup$ As commented above, when you cross the international date line the date changes, but it also changes at midnight, so as you circle the globe at speed the date steps forward (or back) as you cross the dateline, but steps in the opposite direction as you 'pass' midnight. So if local time was midday Monday as you cross the dateline, and now you are at midday on Tuesday, but as you continue to travel the time of day reduces till you get to the opposite side of the globe it will be midnight, and as you continue you're back into Monday, and when you get back to the dateline its still Monday. $\endgroup$
    – Andy M
    Nov 8, 2023 at 10:04
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The key is recognizing that as you walk around the pole, you not only cross the IDL. You cross all the other time zone boundaries that also converge to the pole. If you are going east so that you set your date back at the IDL, then you are setting your clock time forward hour by hour as you cross the other time zone boundaries. The net result is that the backwards time increment you got at the IDL is balanced out by the forward increments at all the other boundaries, so the only net time change is the (forward) time you took to make the lap. The same holds true, with signs reversed for the boundary crossings, if you walk westwards around the pole. Therefore the IDL may be rendered all the way from pole to pole without creating any apparent "time travel".

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Something to keep in mind about the International Date Line, or any time zone boundary, is that it is a human construct meant to help delineate an area of equal time for the local residents. I'm not exactly sure what you are asking in your question, but traveling only requires that you change your clock when you reach your destination, not during transit. It should be noted that the time zone map in Antarctica is already adjusted to account for the "polar time" phenomenon you are worried about. See below, where it shows the polar area using one time zone. I don't think the North Pole has anything like this, because there is no land or "local area", so sailors can just use whatever time zone they wish.

Map of approximate time zones on the continent of Antarctica. Stations could use time zones not matching the map. From Wikipedia

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