What Is the International Date Line? How It Works and Why It Bends
The International Date Line (IDL) is the imaginary line on Earth where one calendar day ends and the next begins. Stand on one side and it is Tuesday. Cross it, and suddenly it is Wednesday -- or Monday, depending on which direction you go.
It runs roughly along the 180 degree meridian in the Pacific Ocean. But it does not follow a straight line -- it zigzags to keep countries and island groups on the same calendar day.
Crossing the IDL: East vs West
- Flying west (Asia to Americas): you gain a day. Monday becomes Tuesday.
- Flying east (Americas to Asia): you lose a day. Tuesday becomes Monday.
This is why a flight from Tokyo to Los Angeles that departs Monday evening arrives Monday morning -- you "skip" a day going east.
Why the IDL Bends
If the IDL followed 180 degrees exactly, it would split Russia and several island nations into two different calendar days. Instead, the line makes several detours: an eastward bulge past Russia, a westward dip around Kiribati (which moved the IDL eastward in 1995), and a westward dip around Fiji and Tonga to keep them aligned with Australia and New Zealand business days.
The Samoa Time Zone Change
In 2011, Samoa made a dramatic move: it switched from the eastern side of the IDL to the western side, skipping an entire calendar day (December 30, 2011 simply did not exist in Samoa). The reason was economics -- Samoa's main trading partners are Australia and New Zealand, not the US.
First and Last Places to See Each Day
The first places to greet a new calendar day are Kiribati's Line Islands (GMT+14, the earliest time zone in the world) and Samoa/Tonga (GMT+13). The last places are Baker Island (GMT-12, uninhabited), American Samoa (GMT-11), and Hawaii (GMT-10). The maximum calendar day difference on Earth is 26 hours.