CALENDARS, perhaps man's most ambitious attempt to control time, are predicated on three astronomical certainties: the earth spinning on its axis (a day); the time it takes for the moon to circle the earth (a month), and the approximate time it takes the earth to revolve around the sun (a year). In 46 B.C., Emperor Julius Caesar borrowed from Egyptian and Jewish calendars by instituting a solar year of a dozen 30-day months, with five days left over and a leap year every four years.

But Caesar miscalculated, and over time the 11-minute annual discrepancy between his calendar and the solar year had accumulated a debit of 10 days. By the 16th century, the spring equinox--and Easter, a centerpiece of the Christian religion, which was linked to it--had begun to drift backward from its March mooring into winter. In 1582, Pope Gregory XIII assembled a committee, including the influential Jesuit mathematician Christopher Clavius, and issued a Papal Bull, creating our present-day Christian calendar. New Year's Day was restored to January 1 after more than 1,000 years of being celebrated in late March. There would be no leap years in centesimal years, except those divisible by 400. And, in his most extraordinary move, to anchor Easter, Gregory scissored 10 days off the Julian calendar.

On the night of October 4, 1582, people went to bed as usual; they awoke to find it was October 15--11 days later. While Roman Catholic countries adopted the modifications at once, Protestant England and the Colonies only came around in 1752. A footnote: The Gregorian calendar, one of 40 active calendars in the world, is still not entirely accurate. It runs 26 seconds fast a year, leaving a margin of error of six days every 10,000 years. So don't look back--the next millennium is gaining on us.