Calendars based on the movements of the sun and moon, have been used since
ancient times, but none has been perfect. The Julian calendar, under which Western
nations measured time until AD 1582, was authorized by Julius Caesar in 46 BC, the year
709 of Rome. His expert was a Greek, Sosigenes. The Julian calendar, on the assumption
that the true year was 365 ¼ days, gave every fourth year 366 days. The Venerable Bede,
an Anglo-Saxon monk, announced in AD 730 that the 365 ¼-day Julian year was 11 min,
14 sec too long, accumulative error of about a day every 128 years, but nothing was done
about it for more than 800 years.

By 1582 the accumulated error was estimated to amount to 10 days. In that year Pope
Gregory XIII decreed that the day following Oct. 4, 1582, should be called Oct. 15, thus
dropping 10 days and initiating what became known as the Gregorian calendar.
However, with common years 365 days and a 366-day leap year every fourth year, the
error in the length of the year would have recurred at the rate of a little more than 3 days
every 400 years. Therefore, 3 of every 4 centesimal years (years ending in 00) were made
common years, not leap years. Thus, 1600 was a leap year; 1700, 1800, and 1900 were
not, but 2000 was. Leap years are those years divisible by 4, except centesimal years,
which are common unless divisible by 400. France, Italy, Spain, Portugal, and
Luxembourg adopted the Gregorian calendar at once. Within 2 years most German
Catholic states, Belgium, and parts of Switzerland and the Netherlands were brought
under the new calendar, and Hungary followed in 1587. The rest of the Netherlands,
along with Denmark and the German Protestant states, made the change in 1699-1700
(German Protestants retained the old reckoning of Easter until 1776).

The British government imposed the Gregorian calendar on all its possessions,
including the American colonies, in 1752. The British decreed that the day following
Sept. 2, 1752, should be called Sept. 14, a loss of 11 days. All dates preceding were
marked O.S., for Old Style. In addition, New Year’s Day was moved to Jan. 1 from Mar.
25 (e.g., under the old reckoning, Mar. 24, 1700, had been followed by Mar. 25, 1701).
George Washington’s birth date, which was Feb. 11, 1731, O.S., became Feb. 22, 1732,
New Style (N.S.). In 1753 Sweden too went Gregorian, retaining the old Easter rules until
1844.

In 1793 the French revolutionary government adopted a calendar of 12 months of 30
days each with 5 extra days in September of each common year and a 6th extra day every
4th year. Napoleon reinstated the Gregorian calendar in 1806.

The Gregorian system later spread to non-European regions. First in the European
colonies and then in the independent countries, replacing traditional calendars at least for
official purposes. Japan in 1873, Egypt in 1875, China in 1912, and Turkey in 1917 made
the change, usually in conjunction with political upheavals. In China, the republican
government began reckoning years from its 1911 founding — e.g., 1948 was designated
the year 37. After 1949, the Communists adopted the Common, or Christian Era, year
count, even for the traditional lunar calendar.

In 1918 the revolutionary government in the Soviet Union decreed that the day after
Jan. 31, 1918, O.S., would become Feb. 14, 1918, N.S. Greece followed in 1923. (The
Russian Orthodox Church has retained the Julian calendar, as have various Middle
Eastern Christian sects.) For the first time in history, all major cultures have one calendar.

To convert from the Julian to the Gregorian calendar, add 10 days to dates Oct. 5,
1582, through Feb. 28, 1700; after that date add 11 days through Feb. 28, 1800; 12 days
through Feb. 28, 1900; and 13 days through Feb. 28, 2100. A century consists of 100
consecutive calendar years. The 1st century AD consisted of the years 1 through 100. The
20th century consists of the years 1901 through 2000 and will end Dec. 31, 2000. The 21st
century began Jan. 1, 2001.