calendar

This collection of functions implements a calendar window.  It
generates a calendar for the current month, together with the
previous and coming months, or for any other three-month period.
The calendar can be scrolled forward and backward in the window to
show months in the past or future; the cursor can move forward and
backward by days, weeks, or months, making it possible, for
instance, to jump to the date a specified number of days, weeks, or
months from the date under the cursor.  The user can display a list
of holidays and other notable days for the period shown; the
notable days can be marked on the calendar, if desired.  The user
can also specify that dates having corresponding diary entries (in
a file that the user specifies) be marked; the diary entries for
any date can be viewed in a separate window.  The diary and the
notable days can be viewed independently of the calendar.  Dates
can be translated from the (usual) Gregorian calendar to the day of
the year/days remaining in year, to the ISO commercial calendar, to
the Julian (old style) calendar, to the Hebrew calendar, to the
Islamic calendar, to the Bahá’í calendar, to the French
Revolutionary calendar, to the Mayan calendar, to the Chinese
calendar, to the Coptic calendar, to the Ethiopic calendar, and to
the astronomical (Julian) day number.  Times of sunrise/sunset can
be displayed, as can the phases of the moon.  Appointment
notification for diary entries is available.  Calendar printing via
LaTeX is available.

The following files are part of the calendar/diary code:

   appt.el                    Appointment notification
   cal-bahai.el               Bahá’í calendar
   cal-china.el               Chinese calendar
   cal-coptic.el              Coptic/Ethiopic calendars
   cal-dst.el                 Daylight saving time rules
   cal-french.el              French revolutionary calendar
   cal-hebrew.el              Hebrew calendar
   cal-html.el                Calendars in HTML
   cal-islam.el               Islamic calendar
   cal-iso.el                 ISO calendar
   cal-julian.el              Julian/astronomical calendars
   cal-mayan.el               Mayan calendars
   cal-menu.el                Menu support
   cal-move.el                Movement in the calendar
   cal-persia.el              Persian calendar
   cal-tex.el                 Calendars in LaTeX
   cal-x.el                   Dedicated frame functions
   calendar.el                This file
   diary-lib.el               Diary functions
   holidays.el                Holiday functions
   lunar.el                   Phases of the moon
   solar.el                   Sunrise/sunset, equinoxes/solstices

Technical details of all the calendrical calculations can be found in
"Calendrical Calculations: The Ultimate Edition" by Edward M. Reingold
and Nachum Dershowitz, Cambridge University Press (2018).

An earlier version of the technical details appeared in "Calendrical
Calculations" by Nachum Dershowitz and Edward M. Reingold,
Software--Practice and Experience, Volume 20, Number 9 (September, 1990),
pages 899-928 <https://doi.org/10.1002/spe.4380200905>
<https://pdfs.semanticscholar.org/83b1/14f570002a7a8e1d4e3730cd0e4cdbcad212.pdf>,
and in "Calendrical Calculations, Part II: Three Historical Calendars" by
E. M. Reingold, N. Dershowitz, and S. M. Clamen, Software--Practice and
Experience, Volume 23, Number 4 (April, 1993), pages 383-404
<https://doi.org/10.1002/spe.4380230404>
<http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.42.6421&rep=rep1&type=pdf>

A note on how dates are represented:

The standard format for a (Gregorian) calendar date in this file is a
list of integers (MONTH DAY YEAR) -- see the functions
`calendar-extract-year', `calendar-extract-month', and
`calendar-extract-day'.  Internally it also uses an "absolute" format
which is an integer number of days since December 31, 1BC on the
Gregorian calendar (see e.g. `calendar-absolute-from-gregorian'), and
converts between different calendar scales by converting to and from
the absolute format (see e.g. `calendar-iso-from-absolute' in
cal-iso.el).  This representation is also useful for certain
calculations; e.g. `calendar-day-of-week' is simply the absolute
representation modulo 7, because December 31, 1BC is a Sunday.

A note on free variables:

The calendar passes around a few dynamically bound variables, which
unfortunately have rather common names.  They are meant to be
available for external functions, so the names can't be changed.

displayed-month, displayed-year: bound in calendar-generate, the
  central month of the 3 month calendar window
original-date, number: bound in diary-list-entries, the arguments
  with which that function was called.
date, entry: bound in diary-list-sexp-entries (qv)

Bound in diary-list-entries:
diary-entries-list: use in d-l, appt.el, and by diary-add-to-list
diary-saved-point: only used in diary-lib.el, passed to the display func
date-string: only used in diary-lib.el
list-only: don't modify the diary-buffer, just return a list of entries
file-glob-attrs: yuck