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In certain times of the year I can travel 21 hours in time by crossing this 4 km bridge. Where am I?

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    $\begingroup$ I think the question is too broad, because there are many valid answers $\endgroup$
    – Ivo
    Nov 21, 2022 at 8:08
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    $\begingroup$ Made it much more specific. Should be able to find the answer now $\endgroup$ Nov 21, 2022 at 8:22
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    $\begingroup$ Would knowledge or geography be appropriate? $\endgroup$
    – xyldke
    Nov 21, 2022 at 11:41
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    $\begingroup$ For whoever this may concern, the question has been modified in a way that invalidates some answers; it is not recommended and here's some discussion regarding answers that are made invalid by the change (for example kkm's answer). puzzling.meta.stackexchange.com/questions/2925/… $\endgroup$ Nov 21, 2022 at 15:34
  • $\begingroup$ @xyldke added those tags $\endgroup$ Nov 22, 2022 at 2:29

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Another possible answer but didn't found bridge between two place

The islands of Big Diomede and Little Diomede

This is because

Big Diomede is in the easternmost point of Russia and Little Diomede is within the United States' border. They are separated by just two miles (4.8 kilometers) but there is a 20-hour time difference between them.

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  • $\begingroup$ Google tells me another time difference. There might also possible be differences due to different way to do rot13(qnlyvtug fnivat) $\endgroup$
    – xyldke
    Nov 21, 2022 at 11:39
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    $\begingroup$ Previous versions of the question did mention "certain times of the year". $\endgroup$
    – aschepler
    Nov 21, 2022 at 14:00
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    $\begingroup$ You got it! That's the answer I was looking for. $\endgroup$ Nov 22, 2022 at 2:28
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    $\begingroup$ @DmitryKamenetsky, I have been searching the globe around the date line. The 3 hour (45°) of astro time across 5km corresponded to ±89.944⁰ of latitude; no bridges are this close to poles. We deal with the set timezones at high enough latitudes. I found the 2 small islands on both sides of the dateline, but Google Earth showed no bridge; I dropped them off my radar :( The Easternmost TZ in Russia is UTC+12; they don't use DST. +12–21=–9, and the 21h difference is between Russia far East and Alaska far West in HDT=UTC-9 Mar to Nov, certain time of year, check. But where's the bridge??? $\endgroup$ Nov 27, 2022 at 22:04
  • $\begingroup$ @kkm the ice bridge only appears during winter $\endgroup$ Dec 1, 2022 at 14:29
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You're in

Malmö, Sweden,

and, if you cross the

Øresundsbron, a.k.a. Øresund Bridge,

you'll arrive at

the greater Copenhagen area in Denmark.

Because of an outdated historical law, Denmark formally observes mean solar noon time at 15°E longitude, happening to cross the Danish Bornholm island, which is actually further to the East than even Malmö (≈12°E), where you've started your Westward journey! The exact number had been chosen because 15° corresponds to 360°/24, or one hour. This time is close to CEST, UTC+1, but may be offset from it by as much as a second. Beyond that, the UTC is corrected by adding or removing a leap second.

Nobody in fact observes this law, and everyone in Denmark uses CEST precisely. It's interesting, however, that Denmark does not have its own atomic clock in sync with the rest of the world…

Whether it's related to the outdated 19c. law, I don't know.

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  • $\begingroup$ Cool answer, upvoted. I was thinking of another natural bridge $\endgroup$ Nov 20, 2022 at 9:59
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    $\begingroup$ Thanks! Another country which has a similar antiquated law is the UK, naming the 0° Greenwich meridian as the mean solar noon reference, but the UK has a Cesium atomic clock at NPL (Natl Physics Lab), and is formally in sync with the world's time. The UK legal system, like the US and other former colonies and unlike the European one, is much more lenient applying laws on the books. And here's a lifehack for ya: if your world clock app doesn't offer the UTC (e.g., Android doesn't), use Reykjavik time instead. Iceland is using UTC+0 and has no DST, and thus always in sync with the UTC. $\endgroup$ Nov 20, 2022 at 18:39
  • $\begingroup$ Can you elaborate on how this would cause a 21 hour time difference? $\endgroup$ Nov 21, 2022 at 14:55
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    $\begingroup$ @JacobRaihle check the question edit history, when this answer was posted, the question did not mention 21 hours, only "I can travel through time" $\endgroup$ Nov 21, 2022 at 15:29
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Either

the Arctic Bridge from Churchill, Manitoba to Murmansk, Russia

or

the Antarctica Bridge cruise ship

lets you

cross time zones.

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Bering Strait?

This is because

it freezes over during the winter allowing you to cross it on foot, thus traversing the International Date Line.

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Time Zone Bridge over Salmon River?

This is because

The Pacific and Mountain time zones are divided along the Salmon River. Thus, there's actually a place where US 95 crosses the river called Time Zone Bridge. Crossing from one side sends you over into the Pacific Time Zone. Crossing from the other side, you enter Mountain Time..

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