Have you ever flown from the United States to Japan? Or how about to Beijing, China? If so, then you have crossed the International Date Line. Now you may be wondering, “What is the International Date Line?” The International Date Line (IDL) is an imaginary line on the surface of the Earth that runs from the North Pole to the South Pole and determines one calendar day from the next. However, the time difference between either sides of the IDL is not always exactly 24 hours because of local time zone variations. While the world is divided into 24 time zones, there has to be a place where there is a difference in days, somewhere the day truly “starts” on the planet. Thus, the 180° line of longitude, exactly one-half way around the planet from Greenwich, England and 0° longitude is approximately where the IDL is located.
For parts of its length, the IDL follows the meridian of 180° longitude, roughly down the middle of the Pacific Ocean. To avoid crossing nations internally and splitting apart countries in to two days, the line deviates around the far east of Russia and then around various island groups in the Pacific. These various deviations (east or west) generally accommodate the political and/or economic affiliations of the affected areas. In the beginning, tiny Kiribati was split. In 1995 the island country of Kiribati decided to move the IDL. Since the line is simply established by international agreement and there are not treaties or formal agreements associated with the line, most of the rest of the world followed Kiribati and moved the line on their maps. Most recent maps show the change and you’ll see the big panhandle zigzag which keeps Kiribati all within the same day. Now, eastern Kiribati and Hawaii, which are located in the same area of longitude, are a whole day apart.
So how does the IDL affect travel? Without the IDL, people who travel west around the planet would discover that when they returned home, it would seem as though an extra day had passed. This situation actually happened to Magellan’s crew when they returned home after their circumnavigation of the earth.
Here’s how the IDL works when traveling. Let’s say you fly from the United States to Japan. Let’s suppose you leave the United States on Sunday morning. Since you are traveling west, the time advances slowly thanks to time zones and the speed at which your airplane flies, but once you cross the IDL, it is suddenly Monday. On the reverse trip home you fly from Japan to the United States. You leave Japan on Saturday morning but as you cross the Pacific Ocean, the day gets later quickly as you cross time zones moving eastward in an airplane. However, once you cross the International Date Line, the day suddenly changes to Friday.
Crossing the IDL and depending on the direction you are travelling, will have an effect on your per diem. Cross the IDL from the east to the west and a day of per diem is added. Cross the IDL from west to the east and a day of per diem is subtracted. So, in using the example above – traveling from Washington, DC to Japan from Sunday to Saturday – the ETS system that my agency uses calculates per diem like this:
Sunday – $0.00 for lodging and 3/4 M&IE
Monday – Full lodging rate and $0.00 M&IE
Tuesday – Full lodging rate and full M&IE
Wednesday – Full lodging rate and full M&IE
Thursday – Full lodging rate and full M&IE
Friday – Full lodging rate and full M&IE
Saturday – $0.00 for lodging and 3/4 M&IE plus full M&IE for the date that you crossed the IDL and received $0.00 M&IE
Now I personally have not had to cross the IDL. But with my experience as a customer service representative, this topic is highly confusing for travelers, and therefore I have researched it many times as well as have explained it many times. Sometimes travelers will call our agency travel help desk to ask why the ETS system is calculating their per diem incorrectly. What they don’t realize is that they are crossing the IDL and the ETS system is actually calculating their per diem correctly. Other times we receive calls from travelers who are aware they are crossing the IDL, but have no idea how the ETS system is calculating their per diem. They simply just want an explanation how the calculation is being performed. I say ‘simply,’ but most of the time, the IDL is anything but simple.
By: Susan Crouser
“The contents of this message are mine personally and do not reflect any position of the Government or my agency.”