I swear I used to understand this, but my brain no good anymore work.
Say it’s 4 P.M. April 10th right on one side of the date line. Is it 3 P.M. April 11th right on the other? If so, why?
Thanks,
Sua
Here is nice little explanation of the IDL.
Jeff, I could not get that link to work. But Sua, here is a brief explanation.
4PM on this side on April 10. 3PM on the other side on April 11. This is necessary for this reason. I don’t know the number of hours from the IDL to EDL, but say, for purposes of explanation, it is 7. That makes it 11PM on April 10 here in Charleston. Keep adding hours as you go east. Somewhere in the Atlantic, you’re in the next day. So it is April 11 there. I think London is 5 hours ahead of us here in Charleston, so it is 4AM on 4/11. Keep going, and it is 3PM on 4/11 right next to the dateline, back where we changed the dates. If we didn’t change the dates, it would be 3PM on 4/10 if you go west, but 3PM on 4/11 if you go East. Hence, the date change when you go west.
JeffB’s link worked for me. I found it to be accurate, but IMHO far more complicated than it needs to be.
Basically, it’s the interface between the places that are ahead of Greenwich Mean Time and those that are behind it.
Actually, in this thread a while back, dtilque points out that the IDL isn’t really declared by any entity - it simply occurs at the interface between two time zones that are, say, at GMT+12 and GMT-11.
It prevents time travel. 
Imagine you can travel across a time zone in less than an hour. If you go all the way around the world doing this, when you get back to where you started, the time will be earlier than when you left. You could cause logical paradoxes! Oh no! Do it enough times and you could be your own father…
So they adjust the date by a day so this calamitous result is avoided. Now when you get back to where you started, it’s the next day. Saves the universe really.
No. The IDL separates Time Zone M (aka “Mike”) and Time Zone Y (aka “Yankee”). These time zones are only 7.5 degrees wide vice the normal 15 degrees width of the other major time zones (I say “major” because there are minor (1/2 hour offset zones). The reason for that is because it is the same time of day, but different days, in those two time zones. Here’s what the Cryptologic Technician Training Series Module 23 (Time Conversion) [NAVEDTRA A95-23-00-92], prepared by CTRCS RIchard W. Piatt and formerly available from any Navy ESO, has to say about the issue*:
*(Notes from Monty) [sup]1[/sup]Since I can’t remember how to do the indented paragraphs, I’ve used the subquotes feature. [sup]2[/sup]This particular training module is Unclassified.
So, when it’s 4PM on April 11th to the West of the IDL, it’s 4PM on April 10th to the East of the IDL within the twoz zones (M & Y) adjacent to the IDL.
An aside: The US Navy likes to use two different “counting” systems for time. The first is the International Standard having the day go from 0000 (midnight) to 2359 (one minute before midnight) for navigational and message transmission purposes. The second has the day go from 0001 (one minute past midnight) to 2400 (midnight) for charging leave and standing duty watches.
Monty, you make an excellent point - that at many (not all) places, crossing the IDL will change only the date, but not the local time. Local time zones of populated land areas, though, can play havoc with this.
Here is a map of the world, with all (most?) time zones indicated on it, as well as the IDL. On the far right side is the 15[sup]o[/sup] segment of longitude made up by time zones Mike and Yankee.
Out in the open ocean, it appears that what the quoted manual said is quite correct. However, local time zones of the populated land areas don’t necessarily correspond to the 15[sup]o[/sup] slices precisely, and because of this the IDL does a fair amount of zigzagging around.
Not that that sort of thing is unusual anywhere in the world; a glance at the color-codings on the map reveals that quite a few places are considerably “off” from what their time zone “should be” based strictly on longitude. Places can set their local time to be whatever they please.
This map is a close-up of Oceania. Kiribati is split by the 180[sup]o[/sup] line, but by its own decree has looped the IDL what looks to be about 30[sup]o[/sup] to the east. See dtilque’s explanation here.
However, it appears that “time zones”, such as they exist in the open ocean, correspond (fairly) precisely to the 15[sup]o[/sup] longitude slices. I guess the Navy might be more interested in that, anyway.
Brad:
Exactly! The whole time zone thing was purposely designed to account for both the big empty areas (vast tracts of ocean and vast tracts of desert) and the areas which would be better off considered to be in another zone than the one they’d geographically fall into if the zone boundaries just followed the lines of longitude.
The premise posited in the OP of both the time and the date being different when crossing the IDL is what both I, and the Senior Chief I quoted, addressed.
I’m sure others will notice, as you did, that for the areas that don’t follow the longitudinal boundaries, sometimes you cross into a zone which has more than one number-identifier difference than the one you left. I was addressing merely the act of crossing the IDL from M to Y or Y to M zones.
Thanks, Monty. I see the point you were making, and I agree completely.
The date changes at midnight, so visualize the midnight line rotating around the planet once every 24 hours. Midnight is the middle of the night so the line is in the middle of the dark shadow of night moving around the globe. Picture yourself on a Pacific Island just west of the international dateline, standing on the beach at midnight. You are one of the first people on the planet to be in the new day, say January 1, 2000. Meanwhile back in Hawaii on the other side of the date line, it is also night, it is after midnight in the small hours of the morning, but it is December 31. After the midnight line passes over you it continues moving west, bringing January 1 to all the world, including, eventually, Hawaii. After the midnight line has touched the entire planet it has to start again with a new date, January 2. It is internationally established and agreed that this point is the International Date Line in the Pacific.
Your average commercial aircraft doesn’t fly fast enough to keep ahead of the midnight line, so someone with more money than brains could not fly west around the world and drop in on millennium celebrations in many cities at midnight December 31, 1999, but it was possible to celebrate midnight December 31 on the west side of the international dateline, rush to the airport in the dark early morning hours of January 1, fly east towards the sunrise (which is rising on December 31 in, say Hawaii, or Los Angeles) land at the airport and wait for midnight December 31, which you observed on the other side of the dateline, to come rolling in after travelling around the world in the other direction. After observing midnight in Hawaii, you could have gotten back on a plane in the early morning hours of January 1, flown westward across the international dateline to home (following the darkness, away from the January 1 rising sun), landed at home in the dark hours after midnight, and it would be the early morning of January 2nd, due to the international dateline.
Sorry people, I enjoy helping someone visualize something.
If you’re with me so far, then you’ll be able to visualize getting on a plane at a sunny afternoon 4PM April 11 on the east side of the dateline, flying west, following the sun, away from the approaching night, crossing the dateline and landing, that same sunny afternoon or early evening, and it is April 12. Or, on a sunny April 12 afternoon on the west side of the dateline, jumping on a plane and flying towards the approaching night, away from the late afternoon sun, across the International Dateline and landing at sunset on April 11.
The International Dateline is a fixed line that is always one day later on the west side than on the east side but there is a second line, the midnight line, that rotates the planet every 24 hours and is one day later on its east side than it is on its west side. Every time these two lines line up together a new date is initiated.
It’s there to allow dodgy frieght companies in Australia to say things like, “Shipping to the US? We can have your package there THE NEXT DAY!!” Yeah right, technically true I guess, but in my book, it’s still a two-day transit.
It is a 1-900 number to talk to live, hot foreign chicks.
Enjoy yourself. 
Not strictly on-topic, but not a pure hijack either:
In 1990 when my ship finished its WestPac and left Hong Kong for its new homeport, Pearl Harbor, we had an extra day to make up. Traveling by ship you can’t just change your clocks and calendars like on an airplane; you have to experience a day twice. So we went to sleep in the evening of Tuesday, October 16, 1990 and woke up to another Tuesday, October 16, 1990.
If we’d been transiting west instead of east, we’d have had to lose a day, and gone from Monday the 15th directly to Wednesday the 17th.
Another note about the two half-sized time zones straddling the IDL: The reason for this can be seen if you look at the other half of the world. The prime meridian (longitude of 0) runs through London (Greenwitch, actually), but nobody wanted London to be split between two time zones. Hence, the time zone boundaries are 7.5 degrees east and west of zero… And hence also 7.5 degrees east and west of 180.
I was a merchant seaman that sailed back and forth to the Orient from the US west coast several times a year. On the way over we would always skip a day and coming back we had to pay it back.
On one trip we skipped October 30. We spent several weeks in Asia and right before leaving Manila to return to the US I was injured and wound up in the hospital. I then flew back and so wasn’t aboard the day the ship crossed the dateline thus not paying back that day we skipped going over. I’ve felt strange ever since! Am I in some kind of space/time warp? 
I get the feeling the old saying “A day late and a dollar short” takes on greater significance in my life. 