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(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 used by tradition in the Jewish religion.  The
Emacs calendar program uses the Hebrew calendar to determine the dates
of Jewish holidays.  Hebrew calendar dates begin and end at sunset.

   The Islamic calendar is used in many predominantly Islamic countries.
Emacs uses it to determine the dates of Islamic 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 observance 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.

   The Copts use a calendar based on the ancient Egyptian solar
calendar.  Their calendar consists of twelve 30-day months followed by
an extra five-day period.  Once every fourth year they add a leap day
to this extra period to make it six days.  The Ethiopic calendar is
identical in structure, but has different year numbers and month names.

   The Persians use a solar calendar based on a design of Omar Khayyam.
Their calendar consists of twelve months of which the first six have 31
days, the next five have 30 days, and the last has 29 in ordinary years
and 30 in leap years.  Leap years occur in a complicated pattern every
four or five years.

   The Chinese calendar is a complicated system of lunar months arranged
into solar years.  The years go in cycles of sixty, each year containing
either twelve months in an ordinary year or thirteen months in a leap
year; each month has either 29 or 30 days.  Years, ordinary months, and
days are named by combining one of ten "celestial stems" with one of
twelve "terrestrial branches" for a total of sixty names that are
repeated in a cycle of sixty.