Okay, I promised a post on the international date line.
First, some background on time zones. As one travels around the Earth going east, the sun begins to rise later and later. That shift should be one hour for every 15 degrees (or 1000 miles at the equator), which gives us the 5 hour time difference between New York and London, or 14 hours between New York and Japan (which is currently 13 because the US observes Daylight Saving Time and Japan does not).
Okay, now, imagine you were to continuing flying east such that you traveled all the way around the Earth. Sunrise would be approximately the same time it was when you left, but, this is key, you would have gained a day! (If were you to fly west, you would have instead lost a day.)
To avoid mass confusion, we need a date line such that if you were to fly east to Japan (gaining 13 hours) or west (losing 11 hours but gaining a day), you'd be in the locally appropriate date. Arbitrarily, this dateline is roughly at 180 degree longitude, or exactly opposite Greenwich, England. This happens to be a part of the Earth where no one lives (avoiding the problems of locals crossing the dateline during their normal activities).
However, this causes a problem for Judaism. What day we're on is hugely important for perhaps the most fundamental Jewish observance: Shabbat, as well as for countless others. Greenwich, England has absolutely no significance for Jews, and so neither does the secular International Date Line.
If we were to have a "halakhic" date line, it would probably be 180 degrees from the city to which we all direct our daily prayers: Jerusalem. This line is also the lines that marks switch from praying eastward (as I have done my whole life) to praying westward (which is still very disorienting). (I'm ignoring here the issue about great circles vs. Rhumb Lines. Suffice to say that I pray along a Rhumb Line, as does everyone else I know.)
So where would this Jewish date line be? Luckily for me, Japan would still be on the other side of it. Israel is about 30 degrees east of Greenwich, so this date line would be at 150 degrees west. Not many people live on this line, except it would create two problems. One is the Hawaii is now on the opposite side as the rest of United States. The other is that is bisects Alaska. Certainly the US could legally rejigger the line so all of its country is on the east side (as it currently does anyway). Whether that would hold any halakhic weight is a subject for another time.