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The picture below shows an imaginary line on the globe which crosses the Pacific Ocean and works as a rough separator of the Eastern and Western hemispheres.

What is this line called in English? I'm trying to find the history behind its funny shape.

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    $\begingroup$ See this meta question for a discussion on whether or not this question is on-topic. $\endgroup$ – gerrit Jun 23 at 20:15
  • $\begingroup$ I was once tasked to write some software which worked with time and location limits inside time zones. When I got to the IDL I gave up in disgust and said it couldn't be done. Stick to GMT or forget it. $\endgroup$ – RedSonja Jun 24 at 10:42
  • $\begingroup$ @RedSonja Tom feels your pain: youtube.com/watch?v=-5wpm-gesOY $\endgroup$ – Thorbjørn Ravn Andersen Jun 24 at 12:48
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It's the international date line and marks the boundary between the time zones that are +12 and -12 hours from UTC / Greenwich. It should follow the +/-180 degree meridian line, but zigs and zags to include territories or islands within a "day" thus the Aleutians islands are in the same time zone as the Hawaiian islands.

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    $\begingroup$ Kiribati has the most extreme timezone: UTC+14. That's why there's that weird hook in the International Date Line. $\endgroup$ – CJ Dennis Jun 23 at 23:02
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    $\begingroup$ Good answer, but it might be better to say that the IDL separates time zones ahead of UTC vs. those behind it, rather than specifically +12 from -12, since there are several exceptions where the numbers aren't actually +12 and -12. For example, -10 UTC actually touches +14 UTC along the IDL between Kiribati and French Polynesia. +12 touches -9 along the Bering Straight. $\endgroup$ – reirab Jun 24 at 5:22
  • $\begingroup$ Wow. This is a big thing! $\endgroup$ – Igor Soloydenko Jun 24 at 16:16
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Mkennedy's answer is right at identifying that line as the international date line and explaining how it works. However, I'd like to add on the main weird points of the line. North to south:

In the Bering sea, the line bends eastward to include all Russia in the same side, and then it bends westward to include the Aleutian islands on the same side as Alaska and the rest of the United States. Interestingly, until the Alaska purchase in 1867, the line was located a lot more to the east to give Alaska the same date than the rest of Russia. Thus, until then the IDL crossed a continent and you could change date just by walking - nowadays the only land crossed by the IDL is Antarctica.

The other weird path in middle of the Pacific Ocean is Kiribati. Despite having a small land surface, Kiribati is a large country when it comes to distance between its extreme points, because it's made of a lot of tiny islands scattered over a large sea area that spans several time zones. Kiribati was formed in 1979 from British and American colonies on both sides of the IDL. It continued to be this way until 1994 when the IDL was shifted eastwards to include the whole country in the same side. Incidentally, that made some islands in eastern Kiribati the first place to welcome the year 2000, which attracted attention and some tourism at that time.

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    $\begingroup$ Although the whole territory is all on the same day, Kiribati still has three time zones. The easternmost is UTC+14. $\endgroup$ – Andrew Leach Jun 22 at 11:26
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    $\begingroup$ Yes, that's the reason that just "some islands" in eastern Kiribati entered year 2000 before anywhere else, and not the whole Kiribati. $\endgroup$ – Pere Jun 22 at 11:44
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    $\begingroup$ There sure are some weird time zones out there. $\endgroup$ – ahiijny Jun 22 at 15:06
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    $\begingroup$ Alaska is 12 hours later than Moscow, so the location of the IDL has no effect on whether Alaska is on the same date as Moscow ;-) $\endgroup$ – gerrit Jun 23 at 8:42
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    $\begingroup$ @gerrit Except between the first and last Sundays in March, when Alaska is on summer time but Moscow isn’t, and so Alaska is 13 hours ahead of Moscow and spends more time in a different date than it would if it was 11 hours behind instead. $\endgroup$ – Mike Scott Jun 23 at 20:38

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