(emacs)Calendar Systems


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Supported Calendar Systems
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   The ISO commercial calendar is used largely in Europe.

   The Julian calendar, named after Julius Caesar, was the one used in
Europe throughout medieval times, and in many countries up until the
nineteenth century.

   Astronomers use a simple counting of days elapsed since noon, Monday,
January 1, 4713 B.C. on the Julian calendar.  The number of days elapsed
is called the *Julian day number* or the *Astronomical day number*.

   The Hebrew calendar is the one used to determine the dates of Jewish
holidays.  Hebrew calendar dates begin and end at sunset.

   The Islamic (Moslem) calendar is the one used to determine the dates
of Moslem holidays.  There is no universal agreement in the Islamic
world about the calendar; Emacs uses a widely accepted version, but the
precise dates of Islamic holidays often depend on proclamation by
religious authorities, not on calculations.  As a consequence, the
actual dates of occurrence can vary slightly from the dates computed by
Emacs.  Islamic calendar dates begin and end at sunset.

   The French Revolutionary calendar was created by the Jacobins after
the 1789 revolution, to represent a more secular and nature-based view
of the annual cycle, and to install a 10-day week in a rationalization
measure similar to the metric system.  The French government officially
abandoned this calendar at the end of 1805.

   The Maya of Central America used three separate, overlapping calendar
systems, the *long count*, the *tzolkin*, and the *haab*.  Emacs knows
about all three of these calendars.  Experts dispute the exact
correlation between the Mayan calendar and our calendar; Emacs uses the
Goodman-Martinez-Thompson correlation in its calculations.


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