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Calendars - The Final Word.

This is the entry from the Encyclopeadia Brittanica On-line:

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                           GREGORIAN CALENDAR,
also called NEW STYLE CALENDAR, solar dating system now in general use. It
was proclaimed in 1582 by Pope Gregory XIII as a reform of the Julian

By the Julian reckoning, the solar year comprised 365 1/4 days; the
intercalation of a "leap day" every four years was intended to maintain
correspondence between the calendar and the seasons. A slight inaccuracy
in the measurement (the solar year comprising more precisely 365 days, 5
hours, 48 minutes, 46 seconds) caused the calendar dates of the seasons to
regress almost one day per century. 

Although this regression had amounted to 14 days by Pope Gregory's time,
he based his reform on restoration of the vernal equinox, then falling on
March 11, to the date (March 21) it had in AD 325, the time of the Council
of Nicaea. The change was effected by advancing the calendar 10 days after
Oct. 4, 1582, the day following being reckoned as October 15. 

The Gregorian calendar differs from the Julian only in that no century
year is a leap year unless it is exactly divisible by 400 (e.g., 1600,
2000). A further refinement, the designation of years evenly divisible by
4,000 as common (not leap) years, will keep the Gregorian calendar
accurate to within one day in 20,000 years. 

Within a year the change had been adopted by the Italian states, Portugal,
Spain, and the German Catholic states. Gradually, other nations adopted
the Gregorian calendar: the Protestant German states in 1699; England and
its colonies in 1752; Sweden in 1753; Japan in 1873; China in 1912; the
Soviet Union in 1918; and Greece in 1923. Muslim countries tend to retain
calendars based on Islam (see Muslim calendar).