(lemacs.info)Other Calendars


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Our Calendar and Other Calendars
--------------------------------

   The Emacs calendar displayed is *always* the Gregorian calendar,
sometimes called the "new style" calendar, which is used in most of the
world today.  However, this calendar did not exist before the sixteenth
century and was not widely used before the eighteenth century; it did
not fully displace the Julian calendar and gain universal acceptance
until the early twentieth century.  This poses a problem for the Emacs
calendar: you can ask for the calendar of any month starting with
January, year 1 of the current era, but the calendar displayed is the
Gregorian, even for a date at which the Gregorian calendar did not
exist!

   Emacs knows about several different calendars, though, not just the
Gregorian calendar.  The following commands describe the date indicated
by point in various calendar notations:

`p c'
     Display ISO commercial calendar equivalent for selected day
     (`calendar-print-iso-date').

`p j'
     Display Julian date for selected day
     (`calendar-print-julian-date').

`p a'
     Display astronomical (Julian) day number for selected day
     (`calendar-print-astro-day-number').

`p h'
     Display Hebrew date for selected day
     (`calendar-print-hebrew-date').

`p i'
     Display Islamic date for selected day
     (`calendar-print-islamic-date').

`p f'
     Display French Revolutionary date for selected day
     (`calendar-print-french-date').

`p m'
     Display Mayan date for selected day (`calendar-print-mayan-date').

   If you are interested in these calendars, you can convert dates one
at a time.  Put point on the desired date of the Gregorian calendar and
press the appropriate keys.  The `p' is a mnemonic for "print" since
Emacs "prints' the equivalent date in the echo area.

   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.

   You can move to dates that you specify on the Commercial, Julian,
astronomical, Hebrew, Islamic, or French calendars:

`g c'
     Move point to a date specified by the ISO commercial calendar
     (`calendar-goto-iso-date').

`g j'
     Move point to a date specified by the Julian calendar
     (`calendar-goto-julian-date').

`g a'
     Move point to a date specified by astronomical (Julian) day number
     (`calendar-goto-astro-day-number').

`g h'
     Move point to a date specified by the Hebrew calendar
     (`calendar-goto-hebrew-date').

`g i'
     Move point to a date specified by the Islamic calendar
     (`calendar-goto-islamic-date').

`g f'
     Move point to a date specified by the French Revolutionary calendar
     (`calendar-goto-french-date').

   These commands ask you for a date on the other calendar, move point
to the Gregorian calendar date equivalent to that date, and display the
other calendar's date in the echo area.  Emacs uses strict completion
(Note: Completion.) whenever it asks you to type a month name, so you
don't have to worry about the spelling of Hebrew, Islamic, or French
names.

   One common question concerning the Hebrew calendar is the computation
of the anniversary of a date of death, called a "yahrzeit."  The Emacs
calendar includes a facility for such calculations.  If you are in the
calendar, the command `M-x list-yahrzeit-dates' asks you for a range of
years and then displays a list of the yahrzeit dates for those years
for the date given by point.  If you are not in the calendar, this
command first asks you for the date of death and the range of years,
and then displays the list of yahrzeit dates.

   Emacs also has many commands for movement on the Mayan calendars.

`g m l'
     Move point to a date specified by the Mayan long count calendar
     (`calendar-goto-mayan-long-count-date').

`g m p t'
     Move point to the previous occurrence of a date specified by the
     Mayan tzolkin calendar (`calendar-previous-tzolkin-date').

`g m n t'
     Move point to the next occurrence of a date specified by the Mayan
     tzolkin calendar (`calendar-next-tzolkin-date').

`g m p h'
     Move point to the previous occurrence of a date specified by the
     Mayan haab calendar (`calendar-previous-haab-date').

`g m n h'
     Move point to the next occurrence of a date specified by the Mayan
     haab calendar (`calendar-next-haab-date').

`g m p c'
     Move point to the previous occurrence of a date specified by the
     Mayan calendar round (`calendar-previous-calendar-round-date').

`g m n c'
     Move point to the next occurrence of a date specified by the Mayan
     calendar round (`calendar-next-calendar-round-date').

   To understand these commands, you need to understand the Mayan
calendars.  The long count is a counting of days with units

1 kin
     = 1  day

1 uinal
     = 20 kin

1 tun
     = 18 uinal

1 katun
     = 20 tun

1 baktun
     = 20 katun

Thus, the long count date 12.16.11.16.6 means 12 baktun, 16 katun, 11
tun, 16 uinal, and 6 kin.  The Emacs calendar can handle Mayan long
count dates as early as 7.17.18.13.1, but no earlier.  When you use the
`g m l' command, type the Mayan long count date with the baktun, katun,
tun, uinal, and kin separated by periods.

   The Mayan tzolkin calendar is a cycle of 260 days formed by a pair of
independent cycles of 13 and 20 days.  Like the haab cycle, this cycle
repeats endlessly, and you can go backward and forward to the previous
or next (respectively) point in the cycle.  When you type `g m p t',
Emacs asks you for a tzolkin date and moves point to the previous
occurrence of that date; type `g m n t' to go to the next occurrence.

   The Mayan haab calendar is a cycle of 365 days arranged as 18 months
of 20 days each, followed a 5-day monthless period.  Since this cycle
repeats endlessly, Emacs lets you go backward and forward to the
previous or next (respectively) point in the cycle.  Type `g m p h' to
go to the previous haab date; Emacs asks you for a haab date and moves
point to the previous occurrence of that date.  Similarly, type `g m n
h' to go to the next haab date.

   The Maya also used the combination of the tzolkin date and the haab
date.  This combination is a cycle of about 52 years called a *calendar
round*.  If you type `g m p c', Emacs asks you for both a haab and a
tzolkin date and then moves point to the previous occurrence of that
combination.  Use `g m p c' to move point to the next occurrence.
Emacs signals an error if the haab/tzolkin date you have typed cannot
occur.

   Emacs uses strict completion (Note: Completion.) whenever it asks
you to type a Mayan name, so you don't have to worry about spelling.


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