Pi may be a universal constant, but only two countries can natively celebrate Pi Day: the U.S. and Belize. That’s because they are the only ones (if Wikipedia is correct) to shorthand their date format so that it can match the first few digits of pi (3.1415), or March 14, 2015.

Most of the rest of the world relies on the day-month-year format, which makes more sense logically because the date elements go from small to big. But this dd-mm-yy format would have pi day fall on the third day of the 14th month of the year. Too bad our Gregorian calendar has only 12 months.

You could regroup the digits and say that Pi Day falls on the 31st day of the fourth month of the year. But nope, that doesn’t work, either: April has only 30 days.

The only other countries that can celebrate a native pi day this century are Kenya and Iran, on April 15, 2031. That’s because they sometimes shorthand their dates as year-month-day, as in 31/4/15.

Political polemicists might see pi day as an example of American exceptionalism, the idea the U.S. differs fundamentally from other nations. But the origin of the date format probably lies in practical matters—specifically, typesetting. Back in the day, people had to line up little blocks of metal lettering and ink them to stamp out newspapers and pamphlets. There’s a Reddit thread anchored by a user named stult who notes that the dd-mm-yy format dominated since the time of Cesar but began to change to mm-dd-yy with the advent of the printing press.

Exactly why the printing press caused the change is unclear, and you’ll find plenty of speculation on the Internet. Perhaps it had to do with the direction in which the type had to be loaded into a frame. If the typefaces had to slide in from left to right, it would have been easier to keep the month letters on the left and the day numbers on the right, where they could be swapped out for the next day’s paper without the need to remove the month letters.

In any case, consider this quirk of a calendar format a bit of good fortune, an excuse to spread some math fun and pi awareness around the world. And, of course, to whip up some pies.


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The Scientific American Pi Day Commemorative Package