What are facts about the International Date Line?

Answer 1

• The IDL was originally defined as the line of longitude at 180 degrees (east and west converge there). This longitude is the other half of the Great Circle that contains the Prime Meridian, also called the Greenwich Meridian.
• There is no international authority that requires any nation to observe the standard defined location of the IDL.
• Some nations including the US and Russia have moved the IDL so that regions near it are technically experiencing the same date, in synch with the ordinary cycles of day and night.
• The island nation of Kiribati famously shifted the IDL 30 degrees west, the equivalent of two time zones, so that it would by definition have the site of land, Caroline Island, that would first experience January 1, 2000, the day popularly and erroneously thought to be the start of the new millennium.
• If you are at a place where the IDL crosses over land [only in Antarctica, I believe] you can straddle the line, and half of you will be in one day, and the other half of you will be in another day.
• If you stand there for any number of days, this will be true every single moment that you stand there, even at midnight, when both of your halves will advance by one day at exactly the same moment.
• However, if at the moment of midnight you pass over the IDL from west to east, you will NOT change dates, but enter the day you were in when you were on the west side.
• You could experience the same calendar day for a full 48 hours before you would have no choice but to enter a later day.