1582: Nobody does anything, anything at all. In fact, nobody does anything whatsoever between Oct. 4 and Oct. 15, 1582, because the 10 intervening days have simply been declared out of existence by the pope. (This offer may not apply outside Italy, Spain and Portugal.)

Where did those days go?

By the mid-1570s, the Julian Calendar established in 45 B.C. was 10 days behind the real seasons of the year. The spring equinox was actually occurring on March 12 or thereabouts, and Easter (set by a formula based on an arbitrary March 22 equinox date) was falling too late in the real springtime.

All this happened because the Earth year is about 11 minutes short of the 365¼ days set by Julius Caesar. It's really 365 days, 5 hours, 48 minutes, 46 seconds. If the drift kept up, Easter would eventually have been observed in the summer, and Christmas in the spring.

So Pope Gregory XIII appointed a commission to tweak the Julian Calendar. Under the leadership of physician Aloysius Lilius and Jesuit astronomer Christopher Clavius, the commission consulted with scientists and clergy. After wrestling with various ideas for half a decade, the commission proposed eliminating three leap years in every 400 (years ending in 00, unless they are divisible by 400).

That would prevent further creep of the calendar against the seasons (except for a minuscule under-correction). But resetting the calendar so the equinox would come in late March needed a more drastic solution: 10 days would have to be skipped, erased, eradicated, obliterated, wiped out of existence.

The commission sent its report to the pope Sept. 14, 1580. He issued a papal bull on Feb. 24, 1582, declaring that the new calendar would go into force in October (when there were few holy days), and that 10 days would be skipped. The day after Oct. 4 would be called not Oct. 5, but Oct. 15.

This was short notice, even for Catholic countries willing to comply with the pope. Only Italy, Spain and Portugal were fully ready by October.

Many people thought their lives were being shortened by 10 days. The pious worried that the saints might not listen to prayers that came 10 days "later" than the traditional saints' days. Everyone's birthday moved to a calendar date 10 days later, too, so 365 days would pass between one birthday and the next. Rents, interest and wages had to be recalculated for a month that had a mere 21 days.

Immediate compliance was spotty, even in Catholic nations. In the German city of Frankfurt, a mob rioted against the pope and his mathematicians. King Henry III of France made the change in December. Parts of the Low Countries jumped from Dec. 21, 1582, directly to Jan. 1, 1583, skipping Christmas.

Most of Catholic Europe adopted the new, Gregorian calendar by 1584. But a continent which was already a patchwork of principalities became in some places a patchwork of calendars. You could travel across a border and go backward or forward 10 days in time. (And you thought crossing the International Date Line was a bother.)

Resistance was strongest in Protestant and Eastern Orthodox lands. The old, Julian calendar held on until 1752 in Britain and its colonies, and right through 1918 in Russia. As a result, the old Soviet Union used to celebrate its own Ten Days That Shook the World � the October Revolution � in November.

Source: Calendar, by David Ewing Duncan

Image: Pope Gregory XIII removed 10 days from the calendar in 1582. Lots of people didn't like it.

Portrait: Lavinia Fontana

This article first appeared on Wired.com Oct. 8, 2008.