The Julian calendar is named for Julius Caesar, who ordered its adoption in 45 B.C. Caesar's calendar, which consisted of 11 months of 30 or 31 days and a 28-day February (extended to 29 days in leap years), was actually quite accurate: it erred from the real solar calendar by only about 11¹/2 minutes a year.

After centuries, though, even a small inaccuracy like this adds up. By the 15th century the Julian calendar had drifted behind the solar calendar by about a week, so that the vernal equinox was falling around March 12 instead of March 20.

In 1582 Pope Gregory XIII ordered another reform of the calendar, which became known as the Gregorian calendar. The date was advanced by 11 days and a new corrective device was introduced: century years such as 1700 or 1800 would no longer be counted as leap years, unless they were (like 1600 or 2000) divisible by 400.

If somewhat inelegant, this system is undeniably effective, and is still in official use in the United States. The Gregorian calendar year differs from the solar year by only 26 seconds—accurate enough, since this only adds up to one day's difference every 3,323 years.