The International Dateline

Have you ever wondered just what the heck that international dateline thing is all about? Well, here's the explanation of your dreams, or your worst nightmares.

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Why do we even need an international dateline? To answer this, you have to first think about time in general and time as it applies to dates. Our day is divided into hours. It takes 24 hours to make a day, and a day is the time the Earth takes to complete one rotation about its axis. You already knew that, didn't you? Okay, but knowing it isn't the same as keeping it in your mind as you consider this international dateline thing. Anyway, as the Earth rotates on its axis, the time at any given place changes. By convention, our day starts at midnight. So, as the Earth rotates, and the location of midnight rotates, the calendar date changes from one day to the next. But, as we shall soon see, we need two places where the date changes.

Why do we need two places where the date changes? The answer is simple. At any given time, we have two dates happening. Yes, we do! Think about it. When it's 12:01 a.m., November 1st, in Chicago, what is the date in New York? It's also November 1st. But what is the date in San Francisco? It's still October 31st, and will be for almost two hours. See? Different places, different dates.

So, we need some way to divide the Earth into two dates. One way is obvious: to the west of midnight it's one date, and to the east of midnight it's the next date. But we still need another dividing line, because it's only midnight at one meridian at a time. We have a line running from one pole to the other, dividing the globe between pre- and post-midnight. But we need another line connecting the poles, to clearly divide the Earth into two dates. That line has been arbitrarily set at the 180 degree meridian, zero degrees being also arbitrarily set at Greenwich, England. The international dateline does not actually follow the 180 degree meridan exactly, but zigs and zags, following political jurisdictions. But for simplicity's sake, we may take it as actually being the 180th meridian. Starting at midnight and going east to the international dateline, the date is one day ahead of the date on the rest of the Earth. One divider is fixed in location, at the international dateline, and the other is moving with the time, at midnight.

Another way to visualize this is to imagine the globe as having two half-circles, each anchored at the poles. One half-circle is fixed to the globe at the international dateline and rotates with it, and the other is at midnight, always opposite the sun. A new date is born when the international dateline passes through midnight. The new date starts out as just a sliver between midnight in the west and the international dateline in the east. As the date gets older, it grows in size until, just before the Earth's rotation brings midnight around again to the international dateline, that date covers nearly the entire globe. For an instant, that date hogs the entire globe. But then immediately a new date begins and the old date begins covering less of the globe, as it is squeezed between midnight, moving to the west, and the international dateline. In 24 hours, the two half-circles cross each other again, and yet another new date is born, as we switch from dates "one" and "two" to dates "two" and "three." 24 hours later, we switch to dates "three" and "four." And so it goes.

From the above explanation, we get an answer to an interesting question: if a day lasts 24 hours, how do we cram two dates onto the globe? The answer is strange, but true. A day lasts 24 hours, but a date lasts 48 hours! Yes, it was November 1st, 1991, somewhere on Earth for 48 hours. Two dates of 48 hours each, divided by 2, gives us our 24 hour day. So, it really does all come out okay.

Perhaps the following is even less interesting than the above, but I'll give it to you anyway. In the definitive history of the Japanese naval air attack on Hawaii on December 7th, 1941, At Dawn We Slept, by Gordon W. Prange, Mr. Prange stated that it is always one day later in Tokyo than at Pearl Harbor, due to the international dateline. This isn't true. When the time is after midnight at Pearl, but not yet midnight in Tokyo, Pearl and Tokyo are "in" the same date. This only lasts for a few hours, until midnight arrives at Tokyo, 60 degrees to the west. In fact, it was this very problem which caused me to look into the whole "international dateline" question. How could Tokyo always be a day ahead of Pearl? The answer was, of course, that Tokyo isn't always a day ahead of Pearl. The otherwise superb Mr. Prange had gotten the international dateline thing wrong.

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I've been getting quite a few e-mail messages that go something like, "My son Johnny is flying to Sydney, Australia. He'll arrive at 10:14 a.m. on the 23rd. What time and date will that be in Philadelphia?" I've been answering these, but I intend to stop. Instead, I'm going to explain how you can figure it out for yourself.

I received this shortcut from Shlomo Liberman:

To calculate different times, the easiest way is to go to time and date.com, click on The World Clock, and create a Personal Page.

Have a comment? Send to rsturge@inreach.com

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