39.9.1 Supported Calendar Systems
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
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 calendar implemented here is the arithmetical Persian calendar
championed by Birashk, based on a 2,820-year cycle. It differs from
the astronomical Persian calendar, which is based on astronomical
events. As of this writing the first future discrepancy is projected
to occur on March 20, 2025. It is currently not clear what the
official calendar of Iran will be that far into the future.
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.