Talk:International Date Line

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Getting the Adjustment Right[edit]

I was in the midst of an edit to be summarized as

Copyedit the marvelous enhancement; move interlang link to top; add'l copyediting

and my edited text was to be

The international date line is an imaginary line that for the most part is at ±180° Longitude, but has an odd shape to pass around Russia and islands in the Pacific. It is on the side of the Earth that lies opposite the prime meridian. Its purpose is to offset the hours that are added as one travels east through each successive time zone.
The first phenomenon to occur in association with the date-line problem was part of Magellan's circumnavigation of the globe. The crew returned to a Spanish stopover on what had to be a Thursday, as attested by various carefully maintained sailing logs. Nevertheless, those on land insisted it was a Wednesday. Although readily understandable, this phenomenon caused great excitement at the time, to the extent that a special delegation was sent to the Pope to explain this oddity to him.
Warning: Wikipedia contains spoilers
The effect of ignoring the date line is also seen in Jules Verne's work of fiction Around the World in Eighty Days, in which the travellers return to London after a trip around the world, thinking that they have lost the bet that is the central premise of the story. Having circumnavigated in the direction opposite Magellan's, they believe the date there to be one day later than what it truly is.
Anyone travelling west and passing the line must add a day to what they would otherwise expect the date and time to be. Correspondingly, those going east must subtract a day. Magellan's crew and Verne's travellers each neglected those adjustments, respectively.

However, i realized at that point that the marvelous account just added by the previous editor (which i touched up as above but treated as essentially accurate) has Magellan's crew (who travelled west) believing in a date that was too advanced, and needing to subtract a day (Thursday minus a day is Wednesday) to get right. I.e., needing to make the same adjustment i asserted Verne's characters did, despite travelling the opposite direction i asserted they did.

Marshalling the Evidence of a Mistake[edit]

  • As you go west, you turn your clock back (make the time you believe in less) by an hour for every 15° of longitude, bcz the sun is not as far toward sunset as you would expect if you made no adjustment. Therefore you have turned back by 24 hours after 360° of westward travel, and to get even with people who didn't travel, you need to "turn your calender" forward (not back) by a day. If your count of days says it's Thursday, the lubbers will agree it's Friday, not Wednesday.
That going round the earth westward was winning one day: upon their return they observed a mismatch of one day between their calendars and those who did not travel, even though they faithfully maintained their ship's log.
"Winning a day" is ambiguous: is it analogous to your "gaining an hour's sleep" when you set the clock back in the fall, or to your clock's "gaining an hour" when you set it forward in the spring? Therefore not evidence either way. But possibly this source of confusion explains the contributor's making what i am calling an error: they misinterpreted "won a day", and assumed the wrong direction for the adjustment the article is describing.
  • I'm a tad embarrassed to admit what the gut-level clincher for me is: i read Verne's story before i was 12 years old, and distinctly remember them landing in San Francisco, continuing east, and finding things getting easier as they got to the more technologized portions of the US, just before their last sea-travel to London. Eastward travel. And the plot falls apart if their calendar needs to be turned forward a day: they think they are a half day late, losing the bet, and it is by turning their calendar back that they realize they've won by a half day margin.

My version:

  • traveling westbound, turn your clock backward when changing time zones, and your calendar forward when crossing the Line
  • traveling eastbound, turn your clock forward when changing time zones, and your calendar backward when crossing the Line

(Unless i'm the confused one, maybe those two formulas belong side by side in the article!)

What to Do[edit]

I'm leaving this apparently erroneous version here, if only bcz i don't know how to fix it: assuming i'm right about the direction of the error,should it be

  • "ship Wednesday, shore Thursday" (error was just reversing the days)
  • "ship Tuesday, shore Wednesday" (error was in deducing ship date from shore one)
  • "ship Thursday, shore Friday" (error was in deducing shore date from ship one)
  • something else (one of the days was chosen at random as a mere example, or both days of week came from mistake in the tricky calculation from Zeller's congruence, or (unlikely, since Spain and Portugal made Julian-to-Gregorian shift while Columbus was at see) using the wrong calendar system.

And avoiding specific days of the week, but referring to the date given in the Magellan article. (I like the reference to days of the week, but let's get agreed about the direction of the correction, then about the days of the week, before putting that back in.)

As to the direction of the correction, i'm leaving my version of Verne as is, and making it vague as to Magellan: better to leave a possible old error in a little longer than to risk going back and forth; the info i'm removing has not been here long enuf for anyone to get used to it. --204.60.201.122 Jerzy 07:02, 14 Jan 2004 (UTC)


A new day is born[edit]

I think the article could mention more clearly that each day springs into excistance at the IDL and then circles the globe westward, as the clock in each timezone reaches 00:00. Ie there are always two "date lines": the IDL and the 23:00/00:00 TZ line. A 3D animation of a globe with timezones changing colors to represent the span of a day would be really fancy.

I'll look into doing this when technology catches up to allow us to make a 3D animated svg. Canadafreakazoid (talk) 04:06, 22 December 2009 (UTC)

Kiribati days[edit]

I updated the number of days that Kirbati's government offices could communicate from three to four per week, as this chart will show is correct:

Asian side/American side Sunday/Saturday Monday/Sunday Tuesday/Monday Wednesday/Tuesday Thursday/Wednesday Friday/Thursday Saturday/Friday

There are four days where each is on a weekday.

Removed from article[edit]

"The Flora Commission, a special U.N. Commission under the direction of B. Joseph Flora of Fair Oaks, California, determined that this inefficiency could no longer be tolerated, as it would serve as an adverse precedent for attempts to increase government efficiency in developing countries." Sole contrib from an anon that I can't verify. Niteowlneils 03:21, 10 Dec 2004 (UTC)

Graphic[edit]

Image:International_Date_Line.png is horrible: asking for the enlargement instead of the thumbnail goes to the image page, which starts out readable with MS IE, but once enuf of it has downloaded, it is too tall for the screen and IE collapses it back to something insignificantly different from the thumbnail. It could only be improved if someone butchered with MS Paint to replace the lettering with something readably large.

--Jerzy·t 30 June 2005 20:56 (UTC)

You have two options: 1) Move the cursor to the lower right-hand corner of the collapsed image, and when the resize icon appears, click it. 2) Go to Tools | Internet Options | Advanced | Multimedia, and uncheck Enable Automatic Image Resizing. — Joe Kress 1 July 2005 05:58 (UTC)

Without irony, Thanks. That was an informative IDL & IE exercise.
But it is still not a substitute for a user-friendly graphic.
--Jerzy·t 1 July 2005 14:55 (UTC)

The graphic also needs a revision to reflect Kiribati's use of the +12, +13, and +14 time zones. Chonak 14:32, 13 August 2006 (UTC)

Tonga to Samoa[edit]

There are several problems with the following paragraph in the article:

The International Date Line can cause confusion among airline travelers. The most troublesome situation usually occurs with short journeys from west to east. For example, to travel from Tonga to Samoa by air takes approximately two hours. Thus, if a person leaves at noon on Tuesday, they will arrive at 2 pm on Monday. Meanwhile, someone in Samoa inquiring about the departing flight may be told there is no flight until the next day. There could also be problems with the traveler having to repeat Monday. Journal entries and photographs may end up out of sequence, and there could be errors in someone's medication schedule. In addition, those making connecting flights might choose the wrong date for the reservation.

  • The paragraph as written is in large part unencyclopedic, and would fit better in a travel guide or something like that. "Problems ... having to repeat Monday"? "Journal entries and photographs ... out of sequence"?
  • According to the map that accompanies the article, Samoa is one hour ahead of Tonga on the clock (leaving aside the calendar date): Tonga is shown as +13 and Samoa as -10. So a two-hour flight that leaves at noon Tuesday would arrive at 3 p.m. Monday local time, not 2 p.m. Never mind, it looks like the -10 notation applies to Tokelau and Samoa is actually -11. Still doesn't make this paragraph much better though.
  • Why is it more troublesome to go west to east and repeat a calendar day than to go east to west and "lose" a day?

I've never travelled between Tonga and Samoa (in either direction) and I don't know if the flight is really two hours, but even if that part is accurate the rest of this paragraph needs a considerable rewrite, if not deletion altogether. I doubt that an airline clerk would be so unaware of the issue as to tell a hapless caller that there's no flight till the next day when the flight in question is actually winging its way over the blue Pacific, but the whole statement is completely unverifiable anyway. There's no question that crossing the Date Line can be confusing for air travellers who haven't done it much, but this isn't a very good way to clear up the confusion. -EDM 23:33, 20 September 2005 (UTC)


First land to the west[edit]

The article claims Caroline Island to be the closest land to the west (and thus the first land to see a new date). The article on the Diomede Islands claims that the International date line passes between Big Diomede island and Little Diomede island at a distance of only 1.25mi from each island. The closest land to the date line on the west would need to be Big Diomede island unless someone can make a claim to knowing of land closer than 1.25mi.

This page [1] support Caroline Island. I'm not convinced. I just think the Diomede islands have been forgotten in the discussion.

Robertbrockway 02:31, 5 February 2006 (UTC)

I can't find any such statement regarding Caroline Atoll, only the statement that it is the easternmost part of Kiribati. An appropriate statement might be that Caroline Atoll is the island farthest east of 180° that is still west of the IDL. The territorial waters east of Caroline Island must be included within Kiribati's time zone and hence the IDL would be at least 12 nautical miles farther east, if not farther. This makes the closest island to the west of the IDL Big Diomede.
But if you include all land, then Antarctica is the closest land west of the IDL. Although de jure time zones are not used in Antarctica, de facto zones are because all stations in Antarctica keep the time of their supply bases. The Amundsen-Scott South Pole Station, which keeps New Zealand time, effectively surrounds the pole—the semicircular taxiway connected to its main aircraft runway actually surrounds the pole [2]. This effectively prevents the IDL from touching the pole. It would require complex wording to describe this weird situation. — Joe Kress 12:03, 9 February 2006 (UTC)
  • The first land to see a new date are the Line Islands, all of them, since they have the highest time zone, UTC+14.  
  • One of those islands is the Caroline Island which is the first land to see the sun a new day (several hours after midnight).  
  • The Big Diomede Island is the land that is closest to the IDL, and the only place you could see land across the line. It has time zone UTC+12 (UTC+13 in the summer).
  • Antarctica has no time zones, except the bases which have de facto time zones. But the bases are so sparsely located in the interior and near the IDL landing so you can't define the placement of the IDL.
  • The IDL ends somewhere in Antarctica but not at a defined place for the same reason, mostly no defined time zone. It does not end at the South pole, which has a time zone for its surroundings. Hope I made things clearer. /BIL 11:40, 29 March 2006 (UTC)
Thanks guys for the great explanation. Makes sense. Robert Brockway 05:37, 13 August 2006 (UTC)
Caroline Island never claimed that it was the closest land to the west of the IDL; if you read the article carefully, it claims that Caroline is the "easternmost land west of the International Date Line" (or did, before Robert removed that statement). -- Seth Ilys 19:48, 13 August 2006 (UTC)
One addition to my list: The first place to see the sun a given day is, during the summer, the Chukotka peninsula (midnight sun) or the Big Diomede Island (almost midnight sun), not sure which place. / BIL 09:59, 14 August 2006 (UTC)

Rephrasing needed?[edit]

Ships must adopt the standard time of a country etc.

Does that imply that a ship's captain must change the clocks on board? Why is this? If I decide to cross the channel without without adjusting my watch, or if I don't adjust it for DST, I'm free to do so. What my watch says also has no impact on the actual time. Even if my watch has it 14:07, if the government says it's noon, it's noon.

So what I think is more logical, is that the time aboard a ship changes, regardless of whether the captain decides to change the clocks. Of course, nowadays with computers and GPS and all that applying the proper time to a ship's clocks may have become easier, but one might imagine that a ship just passing through the territorial waters of some island may not bother to tell its crew to adjust their watches. Shinobu 18:30, 12 December 2006 (UTC)

The reason for the shift is that whenever a ship is within the territorial waters of a another nation but does not plan to stop at one of its ports, it is engaging in "innocent passage". This means that the ship is not subject to the laws of the nation (like being boarded) provided that it obeys all navigational rules of that nation, including communicating with the nation on its own time. If it failed to do so, it would be violating the treaty and would be subject to immediate boarding and seizure. I suppose it is possible that only the captain's clock needs changing, but if the crewmen do not change their watches to match the captain's, serious navigational errors could result. Of course, if the ship plans to dock at a port, all of its clocks and watches must change or there would be total chaos as it tried to unload and load its cargo, including passengers. — Joe Kress 04:55, 13 December 2006 (UTC)

Original research[edit]

The following paragraph is original research:

  • However, even into the 21st century, some maps (including the CIA map of the Standard Time Zones of the World)[1] ignore this Kiribati dateline shift and continue to represent the International Date as a straight line in the Kiribati area.

The source cited does not directly support the claim: it doesn't even mention the Kiribati dateline shift. Jakew 23:34, 31 August 2007 (UTC)

The cited source clearly shows that the date line is a straight line in the Kiribati area. That source is dated 5-07, within the 21st century and long after 1995 when Kiribati adopted a single date. Of course the source does not mention the Kiribati dateline shift, because that is what "ignore the Kiribati dateline shift" means. Hence the statement is not OR. — Joe Kress 06:26, 1 September 2007 (UTC)
"Original research includes editors' personal views, political opinions, and any unpublished analysis or synthesis of published material that appears to advance a position. That is, any facts, opinions, interpretations, definitions, and arguments published by Wikipedia must already have been published by a reliable publication in relation to the topic of the article." (WP:NOR, emph added).
So, if the above is not original research then we should be able to attribute the analysis to a reliable source (we need to be able to say something like: "Dr A. Datelinecommenter comments that 'remarkably, many recent maps still continue to ignore the Kiribati shift'"). Jakew 10:13, 1 September 2007 (UTC)
Source found. Fixed. Jakew 10:34, 1 September 2007 (UTC)
  1. ^ Standard Time Zones of the World by the CIA

Effect on religion[edit]

It has been argued [3] [4] that the International Date Line is a tool of Satan intended to disrupt the correct observation of the Sabbath. I find this highly amusing, but I don't know whether it's appropriate for the main article. Jruderman 10:23, 5 November 2007 (UTC)

Jews have a similar problem with the International Date Line vis-a-vis the Sabbath, although they do not restrict to Jerusalem only, but change at 180° from Jerusalem (possibly faulty memory). — Joe Kress 07:53, 7 November 2007 (UTC)
An international date line can't be avoided. Somewhere there has to be a border between areas with a time zone more than UTC and those less than UTC. The only way to avoid the problem with the correct sabbath day is telling everyone to have the same time zone, e.g. Greenwich or Jerusalem time. Or make the earth flat. At midnight/date shift in Jerusalem there is neccesarily daytime in the Pacific ocean, causing confusing date handling. --BIL (talk) 13:04, 30 April 2008 (UTC)
It is obvious from your two links that correct time zone at least on the Sabbath day is Jerusalem time all over the world. The Sabbath day begins when there is sunset in Jerusalem. --BIL (talk) 13:10, 30 April 2008 (UTC)
Though the first of those links says 'Footnote: For practical purposes, we would presume that the Sabbath would technically begin on the eastern border of the original Promised Land, and progress westward marked by the exact shadow of the earth. Otherwise, there would have been people keeping the Sabbath on separate days, depending on which side of the city (or nation) they lived on. Of course there is no Biblical or historical record of that having occurred.' IOW, sunset at the River Jordan (east of Jerusalem) is slightly earlier than at Jerusalem. Your argument would mean that the sabbath there would begin almost 24 hours after it began at Jerusalem. Alekksandr (talk) 18:52, 15 January 2012 (UTC)


Kiribati and the year 2000--Tonga[edit]

It is not entirely correct that "...Kiribati, by virtue of its easternmost possession, uninhabited Caroline Atoll at 150°25′ west, started the year 2000 on its territory before any other country on earth, ..." Tonga has an odd time zone to begin with--west of the Dateline, but the time itself shifted to one zone east, i.e. earlier.

In order, apparently, to have his country be the first to see the year 2000, the King of Tonga instituted Daylight Saving Time, for the first time ever in that country. Tonga being in the southern hemisphere, January 1 is, technically summer. On the other hand, Daylight Saving Time doesn't really make much sense in the tropics since the length of the days and nights through the year changes less and less as you get nearer the equator.

I seem to remember reading that His Majesty was very annoyed when Kiribati shifted the Dateline within its own territory--presumably for the very practical reason of having the whole country on the same side of the line. But the shift meant that the Caroline Atoll now had the same time as Tonga on Daylight Saving Time. So Tonga was not THE first, but one of the TWO first to see 2000.

I watched much of the television broadcast of the year 2000 arriving in time zones around the world. For the very first arrival of 2000, they had a split screen showing both Tonga and Kiribati. 140.147.160.78 (talk) 18:46, 23 November 2007 (UTC)Stephen Kosciesza

I'm confused -- and I'm probably not alone ...[edit]

Someone who understands this should probably add a paragraph explicitly explaining the following question:

Why isn't the Greenwich Mean Line also the date line?

Isn't that where the date changes (Monday to the east, Sunday to the West)?

I thank you in advance. —Preceding unsigned comment added by 68.9.119.36 (talk) 16:19, 22 March 2008 (UTC)

That would be plain silly, it would mean while it was Monday in Germany it would still be Sunday in Britain, tiotally impractical. Its in the Pacific because few people live there. Thanks, SqueakBox 16:25, 22 March 2008 (UTC)
The Greenwich Meridian is the "middle" of the Earth, and so the "middle" of the day (noon). That means that the other side of the world is midnight-the start of a new day or end of the old day. Or in this case, both. —ScouterSig 16:31, 22 March 2008 (UTC)
It would make sense to have located the Prime Meridian in the same place as the International Date Line. I don't think the anonymous post-er is suggesting that that should be in England, but the opposite, where both lines are in the same place in the middle of the Pacific. I wonder if there's another reason for that other than the obvious fact that politically England was considered (or considered itself) to be the "center" of the world during the time the standards were put in place? --Shubopshadangalang (talk) 23:11, 28 April 2008 (UTC)
The Prime Meridian is located where it is because the United Kingdom used this Prime Meridian, and it became a world standard since it is impractical with several. The United Kingdom was dominant during the time when the decision was made (in 1884 with support from the United States). Read Prime Meridian. The international date line is where it is because the dominant calendar is European and the European colonies wanted to have the same date as Europe as closely as possible. The decision of what time zone and date to have in each colony placed the date line in the Pacific ocean. --BIL (talk) 06:45, 29 April 2008 (UTC)
During the International Meridian Conference, it was indeed proposed that both the Prime Meridian and the International Date Line be located in the middle of the Pacific Ocean, but that proposal was rejected. — Joe Kress (talk) 16:42, 29 April 2008 (UTC)

Explaining the way of dodging the date line could help understand its imaginary nature. For example if New Zealand adopted a new calendar now with leap years in 2010 and then in every 4 years, then they would keep on the west side of the date line until 29 Feb 2010, which would not exist anywhere else, and then between 1 Mar 2010 and 28 Feb 2012 they would be on the east side. Then no 29 Feb 2012 in NZ, and they would be on the west side again, and so on: half the time on the west, half the time on the east. Saj75 (talk) 15:33, 4 December 2008 (UTC)

New External Link?![edit]

Does anyone have aproblem with this http://www.star-k.org/kashrus/kk-trav-dateline.htm It provides extensive information regarding the International Date Line from the religous perspective. —Preceding unsigned comment added by Yoilish (talkcontribs) 19:51, 22 July 2008 (UTC)

Looks good. It should be identified as Jewish datelines. It may even be appropriate to add the info as a separate section. — Joe Kress (talk) 04:27, 23 July 2008 (UTC)

I'm going to follow the idea presented by Joe Kress. Yoilish (talk) 19:47, 24 July 2008 (UTC)

If you add info to the article, there should only be enough to present the basic Jewish ideas without overwhelming the civil meaning. — Joe Kress (talk) 19:53, 24 July 2008 (UTC)

Thoughts for Improvement[edit]

Since some of the following may be controversial, I thought I'd lay them out here rather than simply make the changes.

1) I propose eliminating all references to UTC and its various forms, using only GMT. Reasons: -- the subtle distinctions between GMT and UTC confuse the lay person. They are of no interest to the navigator, as a leap second is lost in the noise if you're using a sextant and dealt with automatically by a GPS. -- the real world (at least those countries that speak English) thinks in terms of GMT -- Greenwich is the basis of both the location of the Date Line and of GMT.

However, if there is strong feeling for UTC, can we at least use only "UTC" and not mention UT1? This is, after all, an article about changing days -- differences of a second, once or twice a year, have no practical meaning to the navigator or traveler.

2) The first section overemphasizes the special cases -- let's start the article simply and move the second paragraph to a less prominent position. I might even argue that the second paragraph belongs in the article on time zones, not here, as it isn't really related to the date line. I don't feel strongly about that, however.

3) I'd like something like this for the first paragraph,

"The International Date Line is the division between time zones at which the time zone on its east is earlier than GMT and that on its west is later. Roughly along 180° longitude, with diversions to pass around nations and other logical groups, it mostly corresponds to the time zone boundary separating +12 and −12 hours GMT. Crossing the IDL traveling east requires changing one's calendar to the previous day, while crossing westbound requires changing to the following day." (links and typography to follow the original)

This emphasizes that the Date Line itself has no effect, that the calendar change comes from crossing from a +12 zone to a -12 zone (or vice versa, ignoring the oddities). Most descriptions, including the current one, imply a sort of magical quality to the IDL, as if it automatically changed the calendar. This is, of course, not the case.

4)I might eliminate the second paragraph of the De facto and De jure section. It is really about time zones, not the IDL.

If there is concensus for keeping it, then I'd like to mark it up to emphasize that as a practical matter all air traffic uses Zulu time (GMT) and most ships do as well for external purposes if on voyages that cross time zones. Internally, ship captains will adjust clocks and calendars at times that are convenient, making an attempt to distribute the benefits of shorter watches (going east) or the nuisance of longer ones (going west) evenly among those who stand watch. They will not (as stated in the Time Zones article) choose midnight for the change if going west -- if you have to lengthen a watch, you do it to day watches, not at night. Jameslwoodward (talk) 13:22, 15 February 2009 (UTC)

After some consideration, I moved the second and third paragraphs to appropriate later sections. I did not delete the second paragraph because it does discuss a change of date on Earth. I dispute that English speaking countries think in terms of GMT. Consider The Official U.S. Time, where all time zones listed are referred to "Coordinated Universal Time", never to "Greenwich Mean Time". The FAA also refers to UTC, explicitly stating that "Zulu" may be used to denote UTC, without mentioning GMT (see the FAA order entitled "Radio and Interphone Communications" - Numbers usage - time). The second paragraph of International Date Line#De facto and de jure date lines is necessary to emphasize that the "nautical date line" is a straight dashed line, not a jagged continuous line. The distinction between UT1 and UTC used to be critical for navigators because the allowed 0.9 second difference amounted to as much as 0.4 km, which could easily mean the difference between a shipwreck on submerged rocks or clearly avoiding them. You are correct that GPS now avoids this problem. I decided to demote GMT for conformity in Wikipedia because every time zone in the list of time zones refers to UTC, not GMT. Nevertheless, I did remove UT1 because GPS now makes the distinction UT1/UTC unnecessary. I also slightly reworded the first sentence to reduce any 'magical' inference. — Joe Kress (talk) 00:54, 19 February 2009 (UTC)

Thanks for your comment. I certainly agree that official references to time refer to UTC -- my comment applies to non-official people, ordinary citizens, who are by far the most likely readers of this. While I don't think Wikipedia should be "dumbed down" for the average reader, I do think we should avoid technical terms when there is a well understood non-technical term that has essentially the same meaning.

Although you've made the point moot by eliminating UT1, I disagree that the difference between UTC and UT1 was ever critical. For starters, there's an immediate +/- half second in reading the chronometer. All of my sextant work has been on small vessels (30-80') and I've never believed that I could do better than around +/- five nautical miles in position. My friends who've used sextants on big ships agree that being within five miles is very good work indeed and takes calm seas and very good visibility. The US Naval Academy's official position is that the reliability of a sextant is +/- three miles. Any navigator who depended on better accuracy would be taking undue risks. I stay more than 0.25 nautical mile (.46km) from submerged rocks even when I think I know very precisely where I am -- too often the chart doesn't know precisely where the rocks are.

A few specifics: - I suggested changing "territories" to "nations". "Territory" has a colonial sense that is not applicable to any of the nations which the IDL bends around -- the USA (Alaska), Kiribati, Samoa, New Zealand, Fiji, Tonga, Tuvalu.

- I wrote my suggestion above a few days ago, "[the IDL] is the division between time zones at which the time zone on its east is earlier than GMT and that on its west is later."

I just looked in my 1966 Bowditch, which defines "date line" (not International) as follows, "The boundary between the (-)12 and (+)12 times zones, corresponding approximately with the 180th meridian."

Perhaps we can agree on the Bowditch definition, which, as I said above, emphasizes that the change in date arises directly from the change in time zone. Bowditch is, of course, in the public domain, so there's no problem there.

- I also prefer "division" to "imaginary line on the surface of the earth", but "boundary", as in Bowditch, would be equally good.

-I prefer the explicitness of "Crossing the IDL traveling east requires changing one's calendar to the previous day" over "traveling east results in a day or 24 hours being subtracted". It's easy for you and me to understand subtracted from what, but such language confuses many people. The confusion is similar to that which arise in going from compass heading to true or the other way -- a routine that is very difficult for many people. Jameslwoodward (talk) 15:53, 19 February 2009 (UTC)

Repeated sentence[edit]

The sentence "During the 1840s, trade interests in the Philippines turned to China, the Dutch East Indies and adjacent areas, and the Philippines changed to Asian dates, that is to the west side of the date line. Monday, 30 December 1844 (ending up as a 365-day year, despite being a leap year) was followed by Wednesday, 1 January 1845" appears twice in the article. (Once at the end of one paragraph, then at the beginning of the paragraph immediately following.) I attempted to remove the redundancy, but the user Quaeler became incredibly upset by this, and restored the repeated sentence. I'll leave it to someone else to remove the redundancy. --75.60.245.204 (talk) 02:46, 2 May 2009 (UTC)

I've deleted the duplicated paragraph. Any text deleted by an anonymous editor, like yourself, without an edit summary is usually reverted because it is usually vandalism. In the future, please give a reason for your edit in the edit summary box. — Joe Kress (talk) 03:59, 2 May 2009 (UTC)

Lead[edit]

I'm adding the following to the end of the lead section to clarify the meaning of the lead section:"Thus, each new day first occurs in Siberia, New Zealand, the Fiji & nearby lands and the middle of the Pacific Ocean.(International Date Line MCMLXXI. Funk & Wagnalls New Encyclopedia, Volume 14, page 131. USA: Funk & Wagnalls, Inc.) --Chuck Marean 02:31, 3 June 2009 (UTC)

I am removing that old (1971) information. Since Kiribati adopted the same date for its entire territory in 1995, a new day always begins in Kiribati, never in Siberia, Fiji, or New Zealand. Even in 1971, that may have been incorrect because Tonga may have already adopted a time zone one hour earlier (GMT+13). Furthermore, that statement gives undue support to the false idea that the IDL is the line that separates the UTC+12 time zone from the UTC−12 time zone. — Joe Kress (talk) 01:25, 4 June 2009 (UTC)

The same day twice?[edit]

That sounds like someone can relive the same day and experiences twice. Reminds me of Groundhog Day. I think that should be reworded, because reliving the same day twice sounds like a lot more than it actually is in the context of this article... 68.200.98.166 (talk) 02:24, 24 December 2009 (UTC)

Done. — Joe Kress (talk) 04:38, 24 December 2009 (UTC)

Help[edit]

The graphics on the page are extraordinarily confusing. The first one has so many lines on it I can't honestly tell where the date line is. Is there any way a new version could be made that makes it clearer where the date "line" actually falls?

Second, though, after looking at the graph of the earth from Polaris, I think I don't actually have the slightest idea what the date line is. Like, if I'm in Dallas, Texas and it's 12:30 in the morning on Tuesday, then it's 11:30 at night (on Monday) in Boise, Idaho. Isn't that where the date line is? I'm so confused. Thanks in advance, hopefully, for your help. Matt Yeager (Talk?) 20:03, 30 April 2010 (UTC)

The first graphic clearly labels the date line with the words "International Date Line" in two places in black to match the date line's color (magnify it by clicking on its thumbnail in the article and then clicking on the image on its own page). I admit the second graphic is confusing. It was just added by another editor—although I immediately modified the wording, it needs more work. — Joe Kress (talk) 08:22, 1 May 2010 (UTC)
Re the Dallas/Boise question: Yes, between the two towns is ONE date line, the moving one. But the view from the top shows, that you inevitably need ANOTHER date line (preferrably a fixed one), where these two date sectors meet at their OTHER ends. And the latter point (or line) is not physically determined, it's an issue of convention, and for that, the IDL was defined. Perhaps the wording in that paragraph could still be made more understandable, but I'm not an english native speaker. --PeterFrankfurt (talk) 01:19, 20 May 2010 (UTC)

The whole "Reason" section, and the schematic of the view from outer space, while no doubt well-intentioned, is superfluous, out of place, and confusing. I've deleted it as well as the later paragraph that belongs in a travel book not an article about time measurement. I did try to change the last sentence of the opening paragraph to capture the gist of the "Reason" section but that part could probably be worded a little better. 66.28.247.235 (talk) 16:52, 20 May 2010 (UTC)

How notable and on-topic is the "Jewish date lines" section?[edit]

I see 5 paragraphs going on and on about Jewish law. This is a religion practiced by 13–14 million people out of a total world population of 6.7 billion. So are FIVE paragraphs truly THAT necessary in a general encyclopedia?

My other objection is that it doesn't seem to be all that on-topic, which is, if you need a reminder, the International Date Line. 63.227.176.102 (talk) 15:53, 21 May 2010 (UTC)

Wikipedia is not a paper encyclopedia, and has no size limits. So there is no reason to remove content just because it's about a minority group. You can, though, if you like, write a summary of the section, move the original section to a new article, and keep only the summary and a "see also" note in the International Date Line article. Perhaps you might call the new article "The date line in Jewish law". Cheers, Unforgettableid (talk) 19:37, 12 September 2010 (UTC)

Blatant advertising?[edit]

There appears to be some advertising at the top of the page. This should probably be removed. —Preceding unsigned comment added by 90.199.13.80 (talk) 23:40, 3 August 2010 (UTC)

Relevant external links are allowed in an External links section at the end of the article. I removed the external link [5] and its paragraph from the lead, but I did not move it to External links because it does not even mention the International Date Line. — Joe Kress (talk) 00:09, 4 August 2010 (UTC)

GMT[edit]

Due to the UK being the only country which still uses it as a reference, all references to GMT have been removed in the interest of parity of esteem. —Preceding unsigned comment added by 86.12.240.223 (talkcontribs) 23:59, 12 September 2010

I am reverting your edit because Denmark insists on refering its time zone to GMT (not UTC) and all ships still use GMT according to the 2011 edition of the Nautical Almanac. By the way, UK actually uses UTC but calls it GMT. — Joe Kress (talk) 06:50, 13 September 2010 (UTC)
Yes and no. AIUI, UK time signals are UTC, but by law UK standard time is GMT. 94.30.84.71 (talk) 11:42, 11 May 2011 (UTC)

UTC replaced GMT as the international civil time standard on 1st January 1972, so how can the tiny countries of UK and Denmark trump the preferences of the rest of the planet? —Preceding unsigned comment added by 86.12.240.223 (talk) 19:08, 14 September 2010 (UTC)

You are unduly fussy; the loudest country of all ignores, for demotic purposes, the International System (SI) of measurement, which is far more important. IMHO, GMT is more widely recognised than UTC by the non-technical, and so should be used except where UTC is necessary. The first reference to GMT and the first reference to Greenwich Mean Time and the first reference to UTC should be links to the corresponding pages. 94.30.84.71 (talk) 11:42, 11 May 2011 (UTC)
When specific cases are mentioned, we must use their time. UTC did not exist in 1884 so only GMT is appropriate there. Ships still prefer GMT according to the 2004 edition of Dutton's Nautical Navigation written by navigators of the United States Navy. It even explains how to convert the UTC broadcast by radio time stations into UT1 (or GMT) by adding the DUT1 correction (tenths of a second) also broadcast by those stations. — Joe Kress (talk) 21:34, 6 October 2010 (UTC)
I doubt whether USN navigators know or care about non-US practice; so, as written, that is a feeble argument. 94.30.84.71 (talk) 11:42, 11 May 2011 (UTC)
GMT is widely used as a synonym for UTC by mariners of most nations and by the laws of many countries. It really ought to be seen as a near synonym for UT1, but the difference today has little practical significance. — Preceding unsigned comment added by 205.178.113.170 (talk) 21:16, 20 October 2011 (UTC)

Dubious -- Latest time[edit]

The latest time is UTC-12, not 11, and that article reports that it has two uninhabited islands.

Time travel[edit]

What if, for example, there exists a plane that could circumnavigate the world in a minute. If a man pilots that plane, and circumnavigates the world 365 times, crossing the international date line 365 times, in "365 minutes", would he have gained or lost a year? My point here being could the convention of gaining/losing time after crossing the date line be broken by sheer velocity? -Iñirante

If a plane travels very fast around the world, and the pilot adjusts his clock according to time zones, then he would put the clock forward one hour per time zone (if going east) and one day back when crossing the International Date Line. The sum of this is zero, the clock would be same as on departure plus the time spent. I think we could write something short about this in the article. This is the very principle behind the IDL concept: If someone travel around the world, then his official date and time is the same at arrival as someone living there. --BIL (talk) 12:24, 19 January 2011 (UTC)

Your hypothetical situation does not belong in the article unless you can find a reliable source that discusses it. No vehicle can circumnavigate the world in a minute, and even those that do so more slowly ignore time zones. Neither commercial airlines [6] nor the space shuttle use time zones—both use UTC or Zulu time, the time at Greenwich, regardless of whatever time zone on Earth's surface they happen to be over. A commercial airline would take many hours to circumnavigate the globe while the space shuttle takes about 90 minutes, both much longer than a minute. Only ocean liners, whose voyages last many days, use time zones. Sandford Fleming, who is credited with inventing world-wide time zones, explicitly proposed a single world-wide time for all purposes for which it would be useful. In 1884, the International Meridian Conference adopted the time at Greenwich as that world-wide time. — Joe Kress (talk) 20:47, 19 January 2011 (UTC)
Well, its hypothetical, as you say. I might just as well have said that the manned vehicle could travel at light speed, because I would have preferred a physics-based explanation. -Iñirante

BIL & Inirante : time zones are administative limits only , when flying at great speed or not and crossing the date line or not , time-sums are never equal or zero since when going west or east you gain or lose on the earth´s rotation against or with your flight track. By using Greenwich time the problem solves itself and automatically the date line (physically the Greenwich anti meridian) gets its definition.80.56.50.56 (talk) 21:30, 20 April 2011 (UTC)

Amelia Earhart[edit]

The article contains remarks about a "Date Line Theory" for Amelia earhart´s breakdown by not arriving at Howland island.This theory (datelinetheory.com)is a nonsense theory : The aircraft was flown on (GMT) Zulu time schedule and "local midnight" and "date line error" (changing almanac pages from July 2 to July 1 and July 2 to July 3 respectively) did not exist on board since the flight (by plan and actually) deployed between 0000 and 2400 GMT , thence within one single day. The writer of the theory has no insight in her subject.80.56.50.56 (talk) 21:10, 20 April 2011 (UTC)

As the above contributor says, the pseudo-science in the cited article at http://www.datelinetheory.com/p/effects-of-date-line.html is nonsense and this section of the WP article should be removed. The article's argument revolves around the possibility of making a significant error in one's position by using the sun's data from the almanac for the wrong day. It is indeed possible to make an error around 60 NM in longitudinal position by using the wrong day in the Almanac for a noon sun sight. However, the last half of the flight was made at night/dawn when a sun sight would have been impossible (sun not visible at night) or impracticable (sun too low in the sky around dawn). The article notes a radio communication from Earhart at 1745 GMT which (according to http://aa.usno.navy.mil/cgi-bin/aa_pap.pl) was dawn at 176 degrees west and a last message at 1845 GMT, which would have been one hour after sunrise, at which point the sun would have been around 12 degrees over the horizon - very low for a reliable fix due to refraction.

Secondly, the article ignores the fact that navigators ignore local time in celestial fixes and calculate LHA (Local Hour Angle) by calculating 360 degrees to the west from GHA (Greenwich Hour Angle) given in the Almanac, thus eliminating the effects of local date or time.

Noonan would indeed have had one difficulty in his calculations - the flight crossed the Equator from Lae at 6°44′S to Howland Island at 0°48′07″N, but perhaps the most significant point is that they were trying to fly 2,200 NM over a featureless sea in a plane with limited fuel range to reach an island which measures all of 450 acres - half the size of Central Park! (data courtesy of Wikipedia). Scartboy (talk) 21:41, 12 August 2011 (UTC)

The following theory disputed above was removed by 205.178.113.170 at 17:53, 15 October 2011‎ with the comment: "Deleted the Earhart tall tale; at minimum it is unvalidated, independent research and thus outside WP standards":

An error in navigation due to the date line has also been suggested as a reason for the disappearance of Amelia Earhart and her navigator Fred Noonan in 1937. Earhart was flying her Lockheed Electra between Lae, New Guinea, and Howland Island on 2 July 1937, when they disappeared shortly after crossing the date line. Noonan may have made a mistake in using the proper dates. An error of this magnitude could have resulted in a 60-nautical-mile (110 km; 69 mi) difference in the relative location of their intended target. In addition, due to the oddities of the date line, Earhart and Noonan are shown to have been alive on the day after they disappeared, flying for several hours on 3 July local time and then disappearing on 2 July. Date line theory: Potential effects of the International Date Line on navigation during Amelia Earhart's 1937 world flight

Joe Kress (talk) 23:37, 15 October 2011 (UTC)

Cultural References[edit]

if Eco is worth a mention, ISTM that Poe should also be mentioned. It will only be necessary to put something like "Edgar Allen Poe's http://en.wikisource.org/wiki/Three_Sundays_in_a_Week (1841)" without much further detail. 94.30.84.71 (talk) 12:01, 11 May 2011 (UTC)

The cultural references section probably should be dropped. The problem with the Poe story is that he was describing the general fact that it can be Sunday in one longitude while it is still Saturday in another (if it's 12:30 am on Sunday in New York City, it is still 11:30 pm on Saturday in Chicago). Poe could have done much better by noting that marine navigators kept "sea days" which ran from noon to noon (it would be Sunday all night long) instead of "civil days" (Sunday all day long) and even distinct from "astronomical days" (Sunday all night long but the next day). Note that none of this is the same as the principle of the date line. In addition, the so-called International Date Line, contrary to popular myth, did not exist when Poe wrote his story. The IDL was invented around 1875 by Joseph Schedler of New York as an educational convenience on maps and globes, as a realization of an "algorithm" for deciding whether a traveller has to change the date on a voyage across the Pacific. Of course the concept long predates his invention. For example, Nathaniel Bowditch (famous as the editor of the New American Practical Navigator) visited Manila in 1796 and noted that the people of Manila kept the date consistent with Mexico. They did not conceive of some line drawn on a map that had to be "crossed" to change the date. When Schedler first drew his IDL, he was unaware that the Philippines had switched date-keeping systems decades earlier so (as shown in the graphic in the article), and the IDL was drawn incorrectly for some years. As late as the Spanish-American War, there was some uncertainty regarding the actual dates of events in the Philippines as reported by the American press. — Preceding unsigned comment added by 205.178.113.170 (talk) 21:11, 20 October 2011 (UTC)
Thanks for the leads. More info is at [7]. Also see the interesting discussion "Where does Sunday begin?" by Editor [W. D. Henkle] of The Ohio Educational Monthly volume 28 (May 1879) 133–141. — Joe Kress (talk) 05:36, 21 October 2011 (UTC)

Historical alterations[edit]

I have a strong recollection of having read, probably in Usenet before December 1996, that in WWII the US forces crossed the existing Date Line, conquered an island, and changed it to US dating thereby moving the Line; and I think that the island eventually reverted. But I have been unable to find a reference to that (the same applies to a UK/US Army Zulu Time joke). If that did indeed happen, it should be mentioned in the Article. 94.30.84.71 (talk) 12:10, 11 May 2011 (UTC)

You may be thinking of Kwajalein, Eniwetok, and Bikini atolls, used for atomic bomb testing after WWII (Kwajalein for support). Since then Kwajalein has been part of a missile test range. I added them to List of time zones by UTC offset#UTC+12:00, M a couple of years ago along with a citation. They were controlled by the Japanese before the war, so the US assumed control as the winner. I assume that during the war, the US military in the Pacific theater may have used either GMT or Hawaii time, but after the war most islands in the Pacific probably reverted to their pre-war time zones. I was thinking of adding them to this article just yesterday, along with the partial rewrite required by the recent news that Samoa will be shifting the line at the end of the year. Kwajalein was requested to change its time zone by the government of the Marshall Islands to conform to the time zones of those islands and did so in 1993. To maintain contact with the continental US, Kwajalein simultaneously changed its work week from Monday thru Friday to Tuesday thru Saturday. They still use Tue-Sat [8]. — Joe Kress (talk) 00:30, 12 May 2011 (UTC)

Kwajalein Missle Range did skip Aug 21 1993, previous to this new year celebrations on the island were known as "Last (dance/ run, etc) on Earth" , as it was the last time zone to switch over to the new year. — Preceding unsigned comment added by 184.88.9.57 (talk) 08:09, 7 June 2011 (UTC)

But to http://catless.ncl.ac.uk/Risks/14.87.html#subj2.1 Risks Digest 14/87, PGN appended : "... Kwajalein, where there was NO Friday, August 20, 1993". 94.30.84.71 (talk) 10:54, 7 June 2011 (UTC)
It could well be Kwajalein. The reversion date fits when I saw it in Usenet. I have read that Samoa will omit Friday December 30th, but not from a trustworthy local source. Thanks. 94.30.84.71 (talk) 17:18, 22 May 2011 (UTC) - - P.S. I've since seen Dec 28 & 31 mentioned. 94.30.84.71 (talk) 17:32, 22 May 2011 (UTC)
Several reliable sources for Samoa are already in the article. The prime minister suggested 29 December 2011, but it has not been codified yet. That would be a Thursday, which seems far removed from the weekend if that was their intent. To move the date line from west to east of an island, a day must be skipped or deleted, which Kwajalein did by deleting Saturday 21 August 1993. They celebrated by holding a two-mile run which began just before midnight Friday evening and ended just after midnight Sunday morning. Conversely, to move the date line from east to west of an island, a day must be repeated, which Samoa did in 1892 when it repeated 4 July, American Independence Day. During that time Samoa had not yet been subdivided by colonial powers. — Joe Kress (talk) 18:05, 23 May 2011 (UTC)
No journalist can be considered reliable, except (usually) when making a direct quote. And most are careless about ambiguity. /// It might be worth including "Western Samoa" or "(Western) Samoa" somewhere in the article (even though the term is formally out of date), to strengthen the distinction between this Samoa and the American one. 94.30.84.71 (talk) 10:54, 7 June 2011 (UTC)
94.30.84.71, could you be more specific regarding a "UK/US Army Zulu Time joke". — Joe Kress (talk) 18:13, 25 May 2011 (UTC)
I cannot recall any part of the wording that would sufficiently help an electronic search engine (I've tried); and a meat-based one with the data should easily recognise it from that. 94.30.84.71 (talk) 10:54, 7 June 2011 (UTC)
I've found an inferior civilian version at http://www.langston.com/Fun_People/1996/1996BHX.html. 94.30.84.71 (talk) 17:08, 7 June 2011 (UTC)
Thanks. LOL!
Wikipedia considers large newspapers like the New York Times as reliable sources, but it doesn't seem to mention this proposal. It may mention legislation enacted later this year. A distinction between (Western) Samoa and American Samoa is indeed desirable, which the sources already mention as a tourist attraction, although the prime minister's statement "'So you can have two birthdays, two weddings and two wedding anniversaries on the same date - on separate days - in less than an hour's flight across - without leaving the Samoan chain," was garbled [9]. — Joe Kress (talk) 21:35, 7 June 2011 (UTC)

Islamic Date Line[edit]

There is a section on the Date Line in Judaism. I suggest a mention of a possible Islamic Date Line. A Web search shows that it is a matter of interest to Muslims, without so far finding any agreed conclusion. 94.30.84.71 (talk) 17:31, 22 May 2011 (UTC)

An International Lunar Date Line was proposed by Mohammad Ilyas of Malaysia in the 1980s [10]. — Joe Kress (talk) 17:44, 25 May 2011 (UTC)
A search will find much more - they are a voluble lot. But it is in the Article that reference should be made; the important point is that readers of the Article should be made aware that (at least) some part of Islam feels a need for such a line. 94.30.84.71 (talk) 11:03, 7 June 2011 (UTC)
Keep in mind that from a day-of-the-week perspective, Islam follows the IDL - i.e. Friday prayers in Hawaii are held ~22 hours after they are held in New Zealand. The discussion of an Islamic Date Line is more to clarify a "line in the sand" where the new lunar month would start. nabeelj (talk) 20:52, 8 August 2011 (UTC)

Ambiguity in Samoan shift 2011[edit]

Article has "Samoa plans to adjust the dateline on December 29, 2011. [1]". It's not clear from that whether they will change immediately after Dec 28 OS, or immediately after Dec 29 OS, or even during Dec 29 OS - if indeed they've actually settled on Dec 29. The article should, when possible, say exactly which whole date, or which 24 hours, will be omitted. 94.30.84.71 (talk) 10:07, 7 June 2011 (UTC)

Unfortunately, that ambiguity resides in the sources available so far. The Herald Sun of Australia has "Prime Minister Tuila'epa Sailele Malielegaoi, who announced the shift would probably take place on December 29", while Metro UK has "pencilled in for December 29". It is only a proposal: The New Zealand Herald stated "Yesterday [6 May 2011] the Government issued a statement confirming the Samoan Cabinet has approved the drafting of legislation to 'enable a change to the Samoa Time Zone'." — Joe Kress (talk) 23:05, 7 June 2011 (UTC)
On Monday 27 June 2011, the Samoan parliament passed into law the date line shift proposal of the prime minister, specifying that Friday 30 December 2011 will be skipped. Tokelau, a New Zealand dependency north of Samoa, "would probably" also skip 30 December.[11][12] Tokelau's parliament, the General Fono, will consider this at its August meeting.[13] I'm revising the article regarding Samoa only. — Joe Kress (talk) 07:21, 11 August 2011 (UTC)
At the end of September, Tokelau's parliament agreed to move west of the date line by skipping 30 December 2011, the same day as Samoa. [14] New Zealand law still must be modified to conform to this change. — Joe Kress (talk) 06:50, 15 October 2011 (UTC)

Animation[edit]

I created this animation: File:Dateline-animation-3deg-borderonly.gif Thumbnails of smaller size result in a static image. Would it be ok to add the animation in its original size, or should I create a 180px wide version? Arnaudf (talk) 11:31, 11 September 2011 (UTC)

File:Crossing Intl Date Line.JPG Nominated for Deletion[edit]

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Nautical Date Line vs. Date line based on land time zones[edit]

The Nautical time date line seems to follow the 180° longitude, except around Samoa, Tonga, New Zealand where it follows the 172½°W longitude (and except near Alaska). Does this mean that the internatonal waters have time zones according to this, and that the eastern Kiribati islands (reaching 151°W longitude) form enclaves of positive time zones and Asian date in the sea of negative time zones and American date? The map at http://aa.usno.navy.mil/faq/docs/world_tzones.php , an offical source, hints this. There seems to one de jure Nautical date line and one de facto Land based date line which do not match. --BIL (talk) 08:40, 19 November 2011 (UTC)

Samoa changing, update needed[edit]

http://www.abc.net.au/news/2011-12-29/samoa-time-zone-jump/3751254 — Preceding unsigned comment added by 27.32.48.153 (talk) 18:59, 29 December 2011 (UTC)

This very interesting story from Samoa got coverage in all the national news papers and stations in Ireland today: http://www.rte.ie/news/2011/1229/samoa.html; http://www.belfasttelegraph.co.uk/news/world-news/samoa-loses-day-in-dateline-switch-16096464.html; http://www.irishtimes.com/newspaper/frontpage/2011/1230/1224309633808.html; http://www.independent.ie/video/video-world-news/samoa-loses-day-in-dateline-switch-2976426.html. This is a very informative article. Well done. 109.77.139.17 (talk) 02:23, 31 December 2011 (UTC)

A Jewish news report[edit]

Sociof, Adam (December 30, 2011). "With Samoa calendar change, question for Jews: When is Shabbat?". JTA, Jewish Telegraphic Agency,. Retrieved 2011-1-1.  DRS (talk) 15:52, 1 January 2012 (UTC)

Image needing update?[edit]

With Samoa's and Tokelau's shift in timezone, is a new image needed? —James (TalkContribs) • 2:50am 16:50, 31 December 2011 (UTC)

Purpose[edit]

I've added a paragraph to clarify the purpose of the line, before we get into travel anomalies:

In conjuction with the moving imaginary line opposite the sun corresponding to midnight, the IDL separates the two calendar days being used on earth at any moment (with an anomaly noted below) except when midnight crosses the IDL.
For the purposes of defining timezones, midnight jumps from timezone to timezone, it doesn't move continuously across the Earth's surface. Therefore timezone related definitions, such as IDL, need to be defined in terms of timezones, not astronomical phenomena. — Preceding unsigned comment added by 88.108.90.98 (talk) 22:00, 2 January 2012 (UTC)

Which makes this paragraph almost superfluous:

The line is necessary in order to have a fixed, albeit arbitrary, boundary on the globe where the calendar date advances in the westbound direction.

--Hugh7 (talk) 20:07, 31 December 2011 (UTC)

I have removed edits by 88.108.90.98. The date does not change between +11 and −11. It changes at the IDL regardless of the time zone differences on either side of it. The arbitrary nature of the Greenwich Meridian is different from that of the IDL. Some neighboring time zones do not differ by one hour. 88.108.90.98 does have a valid point that midnight jumps from one time zone to the next discontinuously, but he did not mention that in his edit. Nor is it necessary to mention this in an article about the IDL. — Joe Kress (talk) 05:32, 3 January 2012 (UTC)
IDL is at the boundary between +11 and -11 time zones. That boundary defines the IDL. Therefore the IDL is defined with reference to those TZs. — Preceding unsigned comment added by 88.108.90.98 (talk) 21:44, 3 January 2012 (UTC)
IDL is not necessarily the boundary between -11 and +11, it is the boundary between the far negative and the far positive times zones, which might be -10, -11, -12, +11, +12, +13 or +14, whatever the adjacent countries have defined. Look at the map. The natical date line is between -12 and +12 (not -11 and +11) but the nautical zones are less interesting since both land areas and ships on high seas choose their time based on that they prefer.--BIL (talk) 09:53, 4 January 2012 (UTC)

Further historical citations relevant to the date line[edit]

Submitted by drs (talk) 13:47, 4 January 2012 (UTC)

The first Christian Missionaries: Chaplain Crawford of the United States Navy, arriving at Samoa, found that the missionaries who first introduced Christianity there, had forgotten to change their reckoning when they crossed the line, and were keeping the " Christian Sabbath" (Sunday) on Saturday.[1]

The Alaskan Transfer: In the February, 1869 edition of the Overland monthly and Out West magazine C. Delavan Bloodgood describes the Alaska transfer event and surrounding circumstances. This is a primary source; a rich account of those times.[2]

The 1884 Prime Meridian Conference: Shelley et al use two pages to describe the Prime Meridian Conference of 1884. They cites Protocols of the Proceedings, 1884.[3][4]

References for this section[edit]

  1. ^ Crafts, Wilbur Fisk (1890). Addresses on the civil Sabbath from a patriotic and humanitarian standpoint (Google eBook). New York, NY: Authors' Publishing Co. p. 28. 
  2. ^ Bloodgood, C. Delavan, U. S. N. (February 1869). "Eight Months at Sitka". Overland monthly and Out West magazine, San Francisco (Making of America Journal Articles, University of Michigan: Humanities Text Initiative) 2 (2): 175–186. Retrieved 2012-January-4. 
  3. ^ Shelley, Fred M.; Archer, J. Clark; Davidson, Fiona M.; Brunn, Stanley D. (1996). Political geography of the United States. New York, NY: Guilford Press. pp. 70,71. ISBN 1-57230-047-7. 
  4. ^ "Protocols of the proceedings". Project Gutenberg. International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October, 1884. Retrieved 2012-January-4. 

Time zone change when crossing the dateline[edit]

The lede currently says

A traveler crossing the IDL eastbound subtracts one day, or 24 hours, so that the the calendar date to the west of the line is repeated. Crossing the IDL westbound results in 24 hours being added, advancing the calendar date by one day.

Likewise, paragraph 4 of the Geography section says

If one flies (or sails) around the world from east to west (the same direction as Magellan's voyage), one hour is lost for every 15° of longitude crossed, losing 24 hours for one circuit of the globe; one compensates by adding 24 hours when crossing the International Date Line from east to west. In contrast, a west-to-east circumnavigation of the globe requires subtracting 24 hours when crossing the IDL. The International Date Line must therefore be observed in conjunction with the Earth's time zones: on crossing it, in either direction, the calendar date is adjusted by one day.

But am I correct that crossing the dateline also moves you into a different time zone? If so, then when crossing it while eastbound one sets the clock back 24 hours because of the dateline but simultaneously sets the clock forward one hour because of the time zone change. So it's misleadingly incomplete to say A traveler crossing the IDL eastbound subtracts one day, or 24 hours; and to say that Crossing the IDL westbound results in 24 hours being added, advancing the calendar date by one day is incomplete in its first part and sometimes wrong in its second part -- e.g. if you cross it westbound from, say, a time of 12:30 AM Wednesday, the net effect on the clock/calendar is plus 24 hours minus 1 hour = +23 hours, so your new time/date is 11:30PM on (still) Wednesday. Duoduoduo (talk) 17:11, 6 January 2012 (UTC)

No, you're not exactly correct, but the confusion is understandable. It depends where you cross the dateline. Look at the map in the main article. At some points, crossing the dateline does indeed change time zones in the way you mean (i.e. from one clock time to another). Siberia to Alaska, for example, is a 3 hour clock time difference. But at other points, the dateline bisects a zone of identical clock time where the offset time (from GMT) is +12 on one side of the dateline and -12 on the other. That is the case between the Marshall Islands and the Baker and Howard Islands.
Since the vast majority of people crossing the dateline these days do so by plane, the simple approach is to consider that at the moment when you cross the dateline, the calendar date is advanced (or regressed) by one but the clock time remains the same. 66.28.247.235 (talk) 17:20, 6 January 2012 (UTC)
I thought that you always change time zone when cross the IDL. Even if you go between UTC-12 and UTC+12 because they are different time zones. Also UTC-11 and UTC+13 are different time zones. --BIL (talk) 12:05, 17 January 2012 (UTC)
No, 66.28.247.235 is right. UTC+12 has the same clock time as UTC-12. For example, at 13:00 UTC it is 01:00 UTC+12 (13+12 mod 24) and it is 01:00 UTC-12 (13-12). Duoduoduo (talk) 16:47, 17 January 2012 (UTC)
So, we have two definitions of "time zone", either "area with same clock time", or "area with same clock time and date" same as "area with same time difference against Greenwich". To have two different definitions for the same term and use them interchanged, is always a big source of confusion. In my opinion, hour and day are both units of time, hour = 1/24 day, and should not be separated. 12 hours is not the same as 1 day and 12 hours. Monday 12:00 is not the same time as Sunday 12:00. A person 6ft 2in is taller than one 5ft 2 in. The summary and conclusion: Moving yourself across the IDL moves yourself into a different time zone, around 24 hours difference. --BIL (talk)

Your definition of time zone is certainly reasonable. But I think it was pretty clear in this thread that the discussion was about whether you change your clock when you cross the dateline. Duoduoduo (talk) 22:39, 17 January 2012 (UTC)

Aircraft[edit]

The article states 'The nautical date line, which is not the same as the International Date Line, is a de jure construction determined by international agreement. It is the result of the 1917 Anglo-French Conference on Time-keeping at Sea, which recommended that all ships, both military and civilian, adopt hourly standard time zones on the high seas.' Does anyone know what the rule is for time on aircraft? Eg suppose that a plane

(a). Leaves American Samoa (UTC minus 11, X) at 1000 UTC on Sunday, when it is 11PM on Saturday local time (so Sunday has not started yet). And when it is 12 midnight on Sunday/Monday in Tokelau.
(b). And flies to Tokelau (UTC+14:00, M†), arriving at say 1100 UTC on Sunday, when it is 1AM on Monday local time (so Sunday has finished). And when it is 12 midnight on Saturday/Sunday in American Samoa.

(Adjust to whatever the normal duration of such a flight is).
And suppose that a passenger dies halfway through the flight. Does the death certificate show the death on

1. Saturday, being the day at the departure point?
2. Sunday, being the day by UTC?
3. Monday, being the day at the arrival point?

Is it relevant which side of 180' the plane was on at the time? Or in which country the plane is registered?

Alekksandr (talk) 22:47, 15 January 2012 (UTC)

Time used by aircraft when communicating with air traffic controllers and navigating is Coordinated Universal Time (UTC) or Zulu time—time zones are not used unless requested. See for example, the FAA Air Traffic Control manual. Times of births and deaths are probably controlled by some international treaty or by the laws of the parents' or passenger's country. — Joe Kress (talk) 03:05, 17 January 2012 (UTC)
If someone is born or dies onboard, that is probably handled document-wise in the country of landing, so its time zone might be used. --BIL (talk) 19:57, 4 February 2012 (UTC)

Copy edits etc.[edit]

I have copy edited the article to make it easier to read. Please check that no oversimplifications or errors have been introduced. Michael Glass (talk) 10:53, 3 February 2012 (UTC)

Still pretty dang confusingly written[edit]

When you first learn physics, they teach you concepts as if relativity didn't exist--only then do they introduce relativity. Can someone who genuinely understands this concept please explain it as if Tonga, etc. didn't exist and only then introduce it and the other exceptions? I think I'm barely starting to understand but it's still really confusing to me. Matt Yeager (Talk?) 17:09, 9 April 2012 (UTC)

This is what made me grasp the concept. Say you start in London and keep heading eastwards around the world, through Europe, Asia, etc. As you head east, you enter different time zones and have to alter your watch: +1 hour in France and western Europe, +2 hours in eastern Europe, +3 hours as you enter Russia, and so on, reaching +12 hours by the time you hit the Pacific islands. If the International Date Line didn't exist, and you kept adding an hour to your watch, by the time you got back to England you'd have wound your watch forwards 24 hours, so it would appear to you that your journey round the world had taken a day longer than it actually had (e.g. your watch would say 1pm Tuesday whereas someone who had been in England all the time you were away travelling would tell you it is 1pm Monday). If you kept circumnavigating the world your watch would get one day ahead with each circuit. The IDL stops this from happening by "taking back" those hourly changes. So heading east around the world you advance your watch by 1 hour 24 times, but on the way round you have to wind it back a whole day, which cancels it out and puts you on the right day. Similarly, if you head west, you have to put your watch back 1 hour 24 times on one journey round the world, but you advance it by a whole day when you cross the IDL. Either way, when you return to the starting point you haven't gained or lost any time. 143.252.80.100 (talk) 15:38, 24 July 2012 (UTC)
Very good. I like the link to modern hourly time zones instead of the mean solar time at evey meridian passed which is not well understood any more. Furthermore, I recently ran across good explanations from the 14th century and immediately after Magellan's remaining ships returned, both stating that if no dateline existed a circumnavigating traveler arriving back home would find that his day of the week and date differed by one day from the day and date kept by those who did not travel, if west than one day earlier and if east than one day later. This is already mentioned in the article in the fourth paragraph of Geography, but should be moved to a more prominent place. — Joe Kress (talk) 19:29, 24 July 2012 (UTC)

Move "Samoan Islands and Tokelau" section after "Eastern Kiribati" section[edit]

The modern IDL move for Samoan Islands and Tokelau occurred in 2011, whereas the IDL move for Eastern Kiribati occurred in 1995. Therefore, let's move the "Samoan Islands and Tokelau" section after "Eastern Kiribati" section. Wideangle (talk) 05:36, 5 July 2013 (UTC)

UTC[edit]

In the second paragraph, the abbreviation UTC is used but is not defined. 87.112.237.138 (talk) 16:21, 6 April 2014 (UTC)