@c This is part of the Emacs manual.
@c Copyright (C) 1985, 86, 87, 93, 94, 95, 1997 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Calendar/Diary, Gnus, Dired, Top
@chapter The Calendar and the Diary
@cindex calendar
@findex calendar
Emacs provides the functions of a desk calendar, with a diary of
planned or past events. To enter the calendar, type @kbd{M-x calendar};
this displays a three-month calendar centered on the current month, with
point on the current date. With a numeric argument, as in @kbd{C-u M-x
calendar}, it prompts you for the month and year to be the center of the
three-month calendar. The calendar uses its own buffer, whose major
mode is Calendar mode.
@kbd{Mouse-2} in the calendar brings up a menu of operations on a
particular date; @kbd{C-Mouse-3} brings up a menu of commonly used
calendar features that are independent of any particular date. To exit
the calendar, type @kbd{q}. @xref{Calendar, Customizing the Calendar
and Diary,, elisp, The Emacs Lisp Reference Manual}, for customization
information about the calendar and diary.
@menu
* Calendar Motion:: Moving through the calendar; selecting a date.
* Scroll Calendar:: Bringing earlier or later months onto the screen.
* Counting Days:: How many days are there between two dates?
* General Calendar:: Exiting or recomputing the calendar.
* LaTeX Calendar:: Print a calendar using LaTeX.
* Holidays:: Displaying dates of holidays.
* Sunrise/Sunset:: Displaying local times of sunrise and sunset.
* Lunar Phases:: Displaying phases of the moon.
* Other Calendars:: Converting dates to other calendar systems.
* Diary:: Displaying events from your diary.
* Appointments:: Reminders when it's time to do something.
* Daylight Savings:: How to specify when daylight savings time is active.
@end menu
@node Calendar Motion
@section Movement in the Calendar
@cindex moving inside the calendar
Calendar mode lets you move through the calendar in logical units of
time such as days, weeks, months, and years. If you move outside the
three months originally displayed, the calendar display ``scrolls''
automatically through time to make the selected date visible. Moving to
a date lets you view its holidays or diary entries, or convert it to other
calendars; moving longer time periods is also useful simply to scroll the
calendar.
@menu
* Calendar Unit Motion:: Moving by days, weeks, months, and years.
* Move to Beginning or End:: Moving to start/end of weeks, months, and years.
* Specified Dates:: Moving to the current date or another
specific date.
@end menu
@node Calendar Unit Motion
@subsection Motion by Standard Lengths of Time
The commands for movement in the calendar buffer parallel the
commands for movement in text. You can move forward and backward by
days, weeks, months, and years.
@table @kbd
@item C-f
Move point one day forward (@code{calendar-forward-day}).
@item C-b
Move point one day backward (@code{calendar-backward-day}).
@item C-n
Move point one week forward (@code{calendar-forward-week}).
@item C-p
Move point one week backward (@code{calendar-backward-week}).
@item M-@}
Move point one month forward (@code{calendar-forward-month}).
@item M-@{
Move point one month backward (@code{calendar-backward-month}).
@item C-x ]
Move point one year forward (@code{calendar-forward-year}).
@item C-x [
Move point one year backward (@code{calendar-backward-year}).
@end table
@kindex C-f @r{(Calendar mode)}
@findex calendar-forward-day
@kindex C-b @r{(Calendar mode)}
@findex calendar-backward-day
@kindex C-n @r{(Calendar mode)}
@findex calendar-forward-week
@kindex C-p @r{(Calendar mode)}
@findex calendar-backward-week
The day and week commands are natural analogues of the usual Emacs
commands for moving by characters and by lines. Just as @kbd{C-n}
usually moves to the same column in the following line, in Calendar
mode it moves to the same day in the following week. And @kbd{C-p}
moves to the same day in the previous week.
The arrow keys are equivalent to @kbd{C-f}, @kbd{C-b}, @kbd{C-n} and
@kbd{C-p}, just as they normally are in other modes.
@kindex M-@} @r{(Calendar mode)}
@findex calendar-forward-month
@kindex M-@{ @r{(Calendar mode)}
@findex calendar-backward-month
@kindex C-x ] @r{(Calendar mode)}
@findex calendar-forward-year
@kindex C-x [ @r{(Calendar mode)}
@findex calendar-forward-year
The commands for motion by months and years work like those for
weeks, but move a larger distance. The month commands @kbd{M-@}} and
@kbd{M-@{} move forward or backward by an entire month's time. The
year commands @kbd{C-x ]} and @w{@kbd{C-x [}} move forward or backward a
whole year.
The easiest way to remember these commands is to consider months and
years analogous to paragraphs and pages of text, respectively. But the
commands themselves are not quite analogous. The ordinary Emacs paragraph
commands move to the beginning or end of a paragraph, whereas these month
and year commands move by an entire month or an entire year, which usually
involves skipping across the end of a month or year.
All these commands accept a numeric argument as a repeat count.
For convenience, the digit keys and the minus sign specify numeric
arguments in Calendar mode even without the Meta modifier. For example,
@kbd{100 C-f} moves point 100 days forward from its present location.
@node Move to Beginning or End
@subsection Beginning or End of Week, Month or Year
A week (or month, or year) is not just a quantity of days; we think of
weeks (months, years) as starting on particular dates. So Calendar mode
provides commands to move to the beginning or end of a week, month or
year:
@table @kbd
@kindex C-a @r{(Calendar mode)}
@findex calendar-beginning-of-week
@item C-a
Move point to start of week (@code{calendar-beginning-of-week}).
@kindex C-e @r{(Calendar mode)}
@findex calendar-end-of-week
@item C-e
Move point to end of week (@code{calendar-end-of-week}).
@kindex M-a @r{(Calendar mode)}
@findex calendar-beginning-of-month
@item M-a
Move point to start of month (@code{calendar-beginning-of-month}).
@kindex M-e @r{(Calendar mode)}
@findex calendar-end-of-month
@item M-e
Move point to end of month (@code{calendar-end-of-month}).
@kindex M-< @r{(Calendar mode)}
@findex calendar-beginning-of-year
@item M-<
Move point to start of year (@code{calendar-beginning-of-year}).
@kindex M-> @r{(Calendar mode)}
@findex calendar-end-of-year
@item M->
Move point to end of year (@code{calendar-end-of-year}).
@end table
These commands also take numeric arguments as repeat counts, with the
repeat count indicating how many weeks, months, or years to move
backward or forward.
@vindex calendar-week-start-day
@cindex weeks, which day they start on
@cindex calendar, first day of week
By default, weeks begin on Sunday. To make them begin on Monday
instead, set the variable @code{calendar-week-start-day} to 1.
@node Specified Dates
@subsection Specified Dates
Calendar mode provides commands for moving to a particular date
specified in various ways.
@table @kbd
@item g d
Move point to specified date (@code{calendar-goto-date}).
@item o
Center calendar around specified month (@code{calendar-other-month}).
@item .
Move point to today's date (@code{calendar-goto-today}).
@end table
@kindex g d @r{(Calendar mode)}
@findex calendar-goto-date
@kbd{g d} (@code{calendar-goto-date}) prompts for a year, a month, and a day
of the month, and then moves to that date. Because the calendar includes all
dates from the beginning of the current era, you must type the year in its
entirety; that is, type @samp{1990}, not @samp{90}.
@kindex o @r{(Calendar mode)}
@findex calendar-other-month
@kbd{o} (@code{calendar-other-month}) prompts for a month and year,
then centers the three-month calendar around that month.
@kindex . @r{(Calendar mode)}
@findex calendar-goto-today
You can return to today's date with @kbd{.}@:
(@code{calendar-goto-today}).
@node Scroll Calendar
@section Scrolling in the Calendar
@cindex scrolling in the calendar
The calendar display scrolls automatically through time when you move out
of the visible portion. You can also scroll it manually. Imagine that the
calendar window contains a long strip of paper with the months on it.
Scrolling it means moving the strip so that new months become visible in
the window.
@table @kbd
@item C-x <
Scroll calendar one month forward (@code{scroll-calendar-left}).
@item C-x >
Scroll calendar one month backward (@code{scroll-calendar-right}).
@item C-v
@itemx @key{NEXT}
Scroll calendar three months forward
(@code{scroll-calendar-left-three-months}).
@item M-v
@itemx @key{PRIOR}
Scroll calendar three months backward
(@code{scroll-calendar-right-three-months}).
@end table
@kindex C-x < @r{(Calendar mode)}
@findex scroll-calendar-left
@kindex C-x > @r{(Calendar mode)}
@findex scroll-calendar-right
The most basic calendar scroll commands scroll by one month at a
time. This means that there are two months of overlap between the
display before the command and the display after. @kbd{C-x <} scrolls
the calendar contents one month to the left; that is, it moves the
display forward in time. @kbd{C-x >} scrolls the contents to the
right, which moves backwards in time.
@kindex C-v @r{(Calendar mode)}
@findex scroll-calendar-left-three-months
@kindex M-v @r{(Calendar mode)}
@findex scroll-calendar-right-three-months
The commands @kbd{C-v} and @kbd{M-v} scroll the calendar by an entire
``screenful''---three months---in analogy with the usual meaning of
these commands. @kbd{C-v} makes later dates visible and @kbd{M-v} makes
earlier dates visible. These commands take a numeric argument as a
repeat count; in particular, since @kbd{C-u} multiplies the next command
by four, typing @kbd{C-u C-v} scrolls the calendar forward by a year and
typing @kbd{C-u M-v} scrolls the calendar backward by a year.
The function keys @key{NEXT} and @key{PRIOR} are equivalent to
@kbd{C-v} and @kbd{M-v}, just as they are in other modes.
@node Counting Days
@section Counting Days
@table @kbd
@item M-=
Display the number of days in the current region
(@code{calendar-count-days-region}).
@end table
@kindex M-= @r{(Calendar mode)}
@findex calendar-count-days-region
To determine the number of days in the region, type @kbd{M-=}
(@code{calendar-count-days-region}). The numbers of days printed is
@emph{inclusive}; that is, it includes the days specified by mark and
point.
@node General Calendar
@section Miscellaneous Calendar Commands
@table @kbd
@item p d
Display day-in-year (@code{calendar-print-day-of-year}).
@item C-c C-l
Regenerate the calendar window (@code{redraw-calendar}).
@item SPC
Scroll the next window (@code{scroll-other-window}).
@item q
Exit from calendar (@code{exit-calendar}).
@end table
@kindex p d @r{(Calendar mode)}
@cindex day of year
@findex calendar-print-day-of-year
To print the number of days elapsed since the start of the year, or
the number of days remaining in the year, type the @kbd{p d} command
(@code{calendar-print-day-of-year}). This displays both of those
numbers in the echo area. The number of days elapsed includes the
selected date. The number of days remaining does not include that
date.
@kindex C-c C-l @r{(Calendar mode)}
@findex redraw-calendar
If the calendar window text gets corrupted, type @kbd{C-c C-l}
(@code{redraw-calendar}) to redraw it. (This can only happen if you use
non-Calendar-mode editing commands.)
@kindex SPC @r{(Calendar mode)}
In Calendar mode, you can use @kbd{SPC} (@code{scroll-other-window})
to scroll the other window. This is handy when you display a list of
holidays or diary entries in another window.
@kindex q @r{(Calendar mode)}
@findex exit-calendar
To exit from the calendar, type @kbd{q} (@code{exit-calendar}). This
buries all buffers related to the calendar, selecting other buffers.
(If a frame contains a dedicated calendar window, exiting from the
calendar iconifies that frame.)
@node LaTeX Calendar
@section LaTeX Calendar
@cindex calendar and La@TeX{}
The Calendar La@TeX{} commands produce a buffer of La@TeX{} code that
prints as a calendar. Depending on the command you use, the printed
calendar covers the day, week, month or year that point is in.
@kindex t @r{(Calendar mode)}
@table @kbd
@item t m
Generate a one-month calendar (@code{cal-tex-cursor-month}).
@item t M
Generate a sideways-printing one-month calendar
(@code{cal-tex-cursor-month-landscape}).
@item t d
Generate a one-day calendar
(@code{cal-tex-cursor-day}).
@item t w 1
Generate a one-page calendar for one week
(@code{cal-tex-cursor-week}).
@item t w 2
Generate a two-page calendar for one week
(@code{cal-tex-cursor-week2}).
@item t w 3
Generate an ISO-style calendar for one week
(@code{cal-tex-cursor-week-iso}).
@item t w 4
Generate a calendar for one Monday-starting week
(@code{cal-tex-cursor-week-monday}).
@item t f w
Generate a Filofax-style two-weeks-at-a-glance calendar
(@code{cal-tex-cursor-filofax-2week}).
@item t f W
Generate a Filofax-style one-week-at-a-glance calendar
(@code{cal-tex-cursor-filofax-week}).
@item t y
Generate a calendar for one year
(@code{cal-tex-cursor-year}).
@item t Y
Generate a sideways-printing calendar for one year
(@code{cal-tex-cursor-year-landscape}).
@item t f y
Generate a Filofax-style calendar for one year
(@code{cal-tex-cursor-filofax-year}).
@end table
Some of these commands print the calendar sideways (in ``landscape
mode''), so it can be wider than it is long. Some of them use Filofax
paper size (3.75in x 6.75in). All of these commands accept a prefix
argument which specifies how many days, weeks, months or years to print
(starting always with the selected one).
If the variable @code{cal-tex-holidays} is non-@code{nil} (the
default), then the printed calendars show the holidays in
@code{calendar-holidays}. If the variable @code{cal-tex-diary} is
non-@code{nil} (the default is @code{nil}), diary entries are included
also (in weekly and monthly calendars only).
@node Holidays
@section Holidays
@cindex holidays
The Emacs calendar knows about all major and many minor holidays,
and can display them.
@table @kbd
@item h
Display holidays for the selected date
(@code{calendar-cursor-holidays}).
@item Mouse-2 Holidays
Display any holidays for the date you click on.
@item x
Mark holidays in the calendar window (@code{mark-calendar-holidays}).
@item u
Unmark calendar window (@code{calendar-unmark}).
@item a
List all holidays for the displayed three months in another window
(@code{list-calendar-holidays}).
@item M-x holidays
List all holidays for three months around today's date in another
window.
@item M-x list-holidays
List holidays in another window for a specified range of years.
@end table
@kindex h @r{(Calendar mode)}
@findex calendar-cursor-holidays
To see if any holidays fall on a given date, position point on that
date in the calendar window and use the @kbd{h} command. Alternatively,
click on that date with @kbd{Mouse-2} and then choose @kbd{Holidays}
from the menu that appears. Either way, this displays the holidays for
that date, in the echo area if they fit there, otherwise in a separate
window.
@kindex x @r{(Calendar mode)}
@findex mark-calendar-holidays
@kindex u @r{(Calendar mode)}
@findex calendar-unmark
To view the distribution of holidays for all the dates shown in the
calendar, use the @kbd{x} command. This displays the dates that are
holidays in a different face (or places a @samp{*} after these dates, if
display with multiple faces is not available). The command applies both
to the currently visible months and to other months that subsequently
become visible by scrolling. To turn marking off and erase the current
marks, type @kbd{u}, which also erases any diary marks (@pxref{Diary}).
@kindex a @r{(Calendar mode)}
@findex list-calendar-holidays
To get even more detailed information, use the @kbd{a} command, which
displays a separate buffer containing a list of all holidays in the
current three-month range. You can use @key{SPC} in the calendar window
to scroll that list.
@findex holidays
The command @kbd{M-x holidays} displays the list of holidays for the
current month and the preceding and succeeding months; this works even
if you don't have a calendar window. If you want the list of holidays
centered around a different month, use @kbd{C-u M-x holidays}, which
prompts for the month and year.
The holidays known to Emacs include United States holidays and the
major Christian, Jewish, and Islamic holidays; also the solstices and
equinoxes.
@findex list-holidays
The command @kbd{M-x list-holidays} displays the list of holidays for
a range of years. This function asks you for the starting and stopping
years, and allows you to choose all the holidays or one of several
categories of holidays. You can use this command even if you don't have
a calendar window.
The dates used by Emacs for holidays are based on @emph{current
practice}, not historical fact. Historically, for instance, the start
of daylight savings time and even its existence have varied from year to
year, but present United States law mandates that daylight savings time
begins on the first Sunday in April. When the daylight savings rules
are set up for the United States, Emacs always uses the present
definition, even though it is wrong for some prior years.
@node Sunrise/Sunset
@section Times of Sunrise and Sunset
@cindex sunrise and sunset
Special calendar commands can tell you, to within a minute or two, the
times of sunrise and sunset for any date.
@table @kbd
@item S
Display times of sunrise and sunset for the selected date
(@code{calendar-sunrise-sunset}).
@item Mouse-2 Sunrise/Sunset
Display times of sunrise and sunset for the date you click on.
@item M-x sunrise-sunset
Display times of sunrise and sunset for today's date.
@item C-u M-x sunrise-sunset
Display times of sunrise and sunset for a specified date.
@end table
@kindex S @r{(Calendar mode)}
@findex calendar-sunrise-sunset
@findex sunrise-sunset
Within the calendar, to display the @emph{local times} of sunrise and
sunset in the echo area, move point to the date you want, and type
@kbd{S}. Alternatively, click @kbd{Mouse-2} on the date, then choose
@kbd{Sunrise/Sunset} from the menu that appears. The command @kbd{M-x
sunrise-sunset} is available outside the calendar to display this
information for today's date or a specified date. To specify a date
other than today, use @kbd{C-u M-x sunrise-sunset}, which prompts for
the year, month, and day.
You can display the times of sunrise and sunset for any location and
any date with @kbd{C-u C-u M-x sunrise-sunset}. This asks you for a
longitude, latitude, number of minutes difference from Coordinated
Universal Time, and date, and then tells you the times of sunrise and
sunset for that location on that date.
Because the times of sunrise and sunset depend on the location on
earth, you need to tell Emacs your latitude, longitude, and location
name before using these commands. Here is an example of what to set:
@vindex calendar-location-name
@vindex calendar-longitude
@vindex calendar-latitude
@example
(setq calendar-latitude 40.1)
(setq calendar-longitude -88.2)
(setq calendar-location-name "Urbana, IL")
@end example
@noindent
Use one decimal place in the values of @code{calendar-latitude} and
@code{calendar-longitude}.
Your time zone also affects the local time of sunrise and sunset.
Emacs usually gets time zone information from the operating system, but
if these values are not what you want (or if the operating system does
not supply them), you must set them yourself. Here is an example:
@vindex calendar-time-zone
@vindex calendar-standard-time-zone-name
@vindex calendar-daylight-time-zone-name
@example
(setq calendar-time-zone -360)
(setq calendar-standard-time-zone-name "CST")
(setq calendar-daylight-time-zone-name "CDT")
@end example
@noindent
The value of @code{calendar-time-zone} is the number of minutes
difference between your local standard time and Coordinated Universal
Time (Greenwich time). The values of
@code{calendar-standard-time-zone-name} and
@code{calendar-daylight-time-zone-name} are the abbreviations used in
your time zone. Emacs displays the times of sunrise and sunset
@emph{corrected for daylight savings time}. @xref{Daylight Savings},
for how daylight savings time is determined.
As a user, you might find it convenient to set the calendar location
variables for your usual physical location in your @file{.emacs} file.
And when you install Emacs on a machine, you can create a
@file{default.el} file which sets them properly for the typical location
of most users of that machine. @xref{Init File}.
@node Lunar Phases
@section Phases of the Moon
@cindex phases of the moon
@cindex moon, phases of
These calendar commands display the dates and times of the phases of
the moon (new moon, first quarter, full moon, last quarter). This
feature is useful for debugging problems that ``depend on the phase of
the moon.''
@table @kbd
@item M
Display the dates and times for all the quarters of the moon for the
three-month period shown (@code{calendar-phases-of-moon}).
@item M-x phases-of-moon
Display dates and times of the quarters of the moon for three months around
today's date.
@end table
@kindex M @r{(Calendar mode)}
@findex calendar-phases-of-moon
Within the calendar, use the @kbd{M} command to display a separate
buffer of the phases of the moon for the current three-month range. The
dates and times listed are accurate to within a few minutes.
@findex phases-of-moon
Outside the calendar, use the command @kbd{M-x phases-of-moon} to
display the list of the phases of the moon for the current month and the
preceding and succeeding months. For information about a different
month, use @kbd{C-u M-x phases-of-moon}, which prompts for the month and
year.
The dates and times given for the phases of the moon are given in
local time (corrected for daylight savings, when appropriate); but if
the variable @code{calendar-time-zone} is void, Coordinated Universal
Time (the Greenwich time zone) is used. @xref{Daylight Savings}.
@node Other Calendars
@section Conversion To and From Other Calendars
@cindex Gregorian calendar
The Emacs calendar displayed is @emph{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. The Emacs calendar can
display any month since 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.
While Emacs cannot display other calendars, it can convert dates to
and from several other calendars.
@menu
* Calendar Systems:: The calendars Emacs understands
(aside from Gregorian).
* To Other Calendar:: Converting the selected date to various calendars.
* From Other Calendar:: Moving to a date specified in another calendar.
* Mayan Calendar:: Moving to a date specified in a Mayan calendar.
@end menu
@node Calendar Systems
@subsection Supported Calendar Systems
@cindex ISO commercial calendar
The ISO commercial calendar is used largely in Europe.
@cindex Julian calendar
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.
@cindex Julian day numbers
@cindex astronomical day numbers
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 @emph{Julian day number} or the @emph{Astronomical day number}.
@cindex Hebrew calendar
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.
@cindex Islamic calendar
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.
@cindex French Revolutionary calendar
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.
@cindex Mayan calendar
The Maya of Central America used three separate, overlapping calendar
systems, the @emph{long count}, the @emph{tzolkin}, and the @emph{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.
@cindex Coptic calendar
@cindex Ethiopic calendar
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.
@cindex Persian calendar
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.
@cindex Chinese calendar
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.
@node To Other Calendar
@subsection Converting To Other Calendars
The following commands describe the selected date (the date at point)
in various other calendar systems:
@table @kbd
@item Mouse-2 Other Calendars
Display the date that you click on, expressed in various other calendars.
@kindex p @r{(Calendar mode)}
@findex calendar-print-iso-date
@item p c
Display ISO commercial calendar equivalent for selected day
(@code{calendar-print-iso-date}).
@findex calendar-print-julian-date
@item p j
Display Julian date for selected day (@code{calendar-print-julian-date}).
@findex calendar-print-astro-day-number
@item p a
Display astronomical (Julian) day number for selected day
(@code{calendar-print-astro-day-number}).
@findex calendar-print-hebrew-date
@item p h
Display Hebrew date for selected day (@code{calendar-print-hebrew-date}).
@findex calendar-print-islamic-date
@item p i
Display Islamic date for selected day (@code{calendar-print-islamic-date}).
@findex calendar-print-french-date
@item p f
Display French Revolutionary date for selected day
(@code{calendar-print-french-date}).
@findex calendar-print-chinese-date
@item p C
Display Chinese date for selected day
(@code{calendar-print-chinese-date}).
@findex calendar-print-coptic-date
@item p k
Display Coptic date for selected day
(@code{calendar-print-coptic-date}).
@findex calendar-print-ethiopic-date
@item p e
Display Ethiopic date for selected day
(@code{calendar-print-ethiopic-date}).
@findex calendar-print-persian-date
@item p p
Display Persian date for selected day
(@code{calendar-print-persian-date}).
@findex calendar-print-mayan-date
@item p m
Display Mayan date for selected day (@code{calendar-print-mayan-date}).
@end table
If you are using X, the easiest way to translate a date into other
calendars is to click on it with @kbd{Mouse-2}, then choose @kbd{Other
Calendars} from the menu that appears. This displays the equivalent
forms of the date in all the calendars Emacs understands, in the form of
a menu. (Choosing an alternative from this menu doesn't actually do
anything---the menu is used only for display.)
Put point on the desired date of the Gregorian calendar, then type the
appropriate keys. The @kbd{p} is a mnemonic for ``print'' since Emacs
``prints'' the equivalent date in the echo area.
@node From Other Calendar
@subsection Converting From Other Calendars
You can use the other supported calendars to specify a date to move
to. This section describes the commands for doing this using calendars
other than Mayan; for the Mayan calendar, see the following section.
@kindex g @var{char} @r{(Calendar mode)}
@findex calendar-goto-iso-date
@findex calendar-goto-julian-date
@findex calendar-goto-astro-day-number
@findex calendar-goto-hebrew-date
@findex calendar-goto-islamic-date
@findex calendar-goto-french-date
@findex calendar-goto-chinese-date
@findex calendar-goto-persian-date
@findex calendar-goto-coptic-date
@findex calendar-goto-ethiopic-date
@table @kbd
@item g c
Move to a date specified in the ISO commercial calendar
(@code{calendar-goto-iso-date}).
@item g j
Move to a date specified in the Julian calendar
(@code{calendar-goto-julian-date}).
@item g a
Move to a date specified in astronomical (Julian) day number
(@code{calendar-goto-astro-day-number}).
@item g h
Move to a date specified in the Hebrew calendar
(@code{calendar-goto-hebrew-date}).
@item g i
Move to a date specified in the Islamic calendar
(@code{calendar-goto-islamic-date}).
@item g f
Move to a date specified in the French Revolutionary calendar
(@code{calendar-goto-french-date}).
@item g C
Move to a date specified in the Chinese calendar
(@code{calendar-goto-chinese-date}).
@item g p
Move to a date specified in the Persian calendar
(@code{calendar-goto-persian-date}).
@item g k
Move to a date specified in the Coptic calendar
(@code{calendar-goto-coptic-date}).
@item g e
Move to a date specified in the Ethiopic calendar
(@code{calendar-goto-ethiopic-date}).
@end table
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
(@pxref{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.
@findex list-yahrzeit-dates
@cindex yahrzeits
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 @kbd{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.
@node Mayan Calendar
@subsection Converting from the Mayan Calendar
Here are the commands to select dates based on the Mayan calendar:
@table @kbd
@item g m l
Move to a date specified by the long count calendar
(@code{calendar-goto-mayan-long-count-date}).
@item g m n t
Move to the next occurrence of a place in the
tzolkin calendar (@code{calendar-next-tzolkin-date}).
@item g m p t
Move to the previous occurrence of a place in the
tzolkin calendar (@code{calendar-previous-tzolkin-date}).
@item g m n h
Move to the next occurrence of a place in the
haab calendar (@code{calendar-next-haab-date}).
@item g m p h
Move to the previous occurrence of a place in the
haab calendar (@code{calendar-previous-haab-date}).
@item g m n c
Move to the next occurrence of a place in the
calendar round (@code{calendar-next-calendar-round-date}).
@item g m p c
Move to the previous occurrence of a place in the
calendar round (@code{calendar-previous-calendar-round-date}).
@end table
@cindex Mayan long count
To understand these commands, you need to understand the Mayan calendars.
The @dfn{long count} is a counting of days with these units:
@display
1 kin = 1 day@ @ @ 1 uinal = 20 kin@ @ @ 1 tun = 18 uinal
1 katun = 20 tun@ @ @ 1 baktun = 20 katun
@end display
@kindex g m @r{(Calendar mode)}
@findex calendar-goto-mayan-long-count-date
@noindent
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
@kbd{g m l} command, type the Mayan long count date with the baktun,
katun, tun, uinal, and kin separated by periods.
@findex calendar-previous-tzolkin-date
@findex calendar-next-tzolkin-date
@cindex Mayan tzolkin calendar
The Mayan tzolkin calendar is a cycle of 260 days formed by a pair of
independent cycles of 13 and 20 days. Since this cycle repeats
endlessly, Emacs provides commands to move backward and forward to the
previous or next point in the cycle. Type @kbd{g m p t} to go to the
previous tzolkin date; Emacs asks you for a tzolkin date and moves point
to the previous occurrence of that date. Similarly, type @kbd{g m n t}
to go to the next occurrence of a tzolkin date.
@findex calendar-previous-haab-date
@findex calendar-next-haab-date
@cindex Mayan haab calendar
The Mayan haab calendar is a cycle of 365 days arranged as 18 months
of 20 days each, followed a 5-day monthless period. Like the tzolkin
cycle, this cycle repeats endlessly, and there are commands to move
backward and forward to the previous or next point in the cycle. Type
@kbd{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 @kbd{g m n h} to go to the next occurrence of a haab
date.
@c This is omitted because it is too long for smallbook format.
@c @findex calendar-previous-calendar-round-date
@findex calendar-next-calendar-round-date
@cindex Mayan calendar round
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
@emph{calendar round}. If you type @kbd{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 @kbd{g m n c} to move point to the
next occurrence of a combination. These commands signal an error if the
haab/tzolkin date combination you have typed is impossible.
Emacs uses strict completion (@pxref{Strict Completion}) whenever it
asks you to type a Mayan name, so you don't have to worry about
spelling.
@node Diary
@section The Diary
@cindex diary
The Emacs diary keeps track of appointments or other events on a daily
basis, in conjunction with the calendar. To use the diary feature, you
must first create a @dfn{diary file} containing a list of events and
their dates. Then Emacs can automatically pick out and display the
events for today, for the immediate future, or for any specified
date.
By default, Emacs uses @file{~/diary} as the diary file. This is the
same file that the @code{calendar} utility uses. A sample
@file{~/diary} file is:
@example
12/22/1988 Twentieth wedding anniversary!!
&1/1. Happy New Year!
10/22 Ruth's birthday.
* 21, *: Payday
Tuesday--weekly meeting with grad students at 10am
Supowit, Shen, Bitner, and Kapoor to attend.
1/13/89 Friday the thirteenth!!
&thu 4pm squash game with Lloyd.
mar 16 Dad's birthday
April 15, 1989 Income tax due.
&* 15 time cards due.
@end example
@noindent
This example uses extra spaces to align the event descriptions of most
of the entries. Such formatting is purely a matter of taste.
Although you probably will start by creating a diary manually, Emacs
provides a number of commands to let you view, add, and change diary
entries.
@menu
* Diary Commands:: Viewing diary entries and associated calendar dates.
* Format of Diary File:: Entering events in your diary.
* Date Formats:: Various ways you can specify dates.
* Adding to Diary:: Commands to create diary entries.
* Special Diary Entries:: Anniversaries, blocks of dates, cyclic entries, etc.
@end menu
@node Diary Commands
@subsection Commands Displaying Diary Entries
Once you have created a @file{~/diary} file, you can use the calendar
to view it. You can also view today's events outside of Calendar mode.
@table @kbd
@item d
Display all diary entries for the selected date
(@code{view-diary-entries}).
@item Mouse-2 Diary
Display all diary entries for the date you click on.
@item s
Display the entire diary file (@code{show-all-diary-entries}).
@item m
Mark all visible dates that have diary entries
(@code{mark-diary-entries}).
@item u
Unmark the calendar window (@code{calendar-unmark}).
@item M-x print-diary-entries
Print hard copy of the diary display as it appears.
@item M-x diary
Display all diary entries for today's date.
@item M-x diary-mail-entries
Mail yourself email reminders about upcoming diary entries.
@end table
@kindex d @r{(Calendar mode)}
@findex view-diary-entries
Displaying the diary entries with @kbd{d} shows in a separate window
the diary entries for the selected date in the calendar. The mode line
of the new window shows the date of the diary entries and any holidays
that fall on that date. If you specify a numeric argument with @kbd{d},
it shows all the diary entries for that many successive days. Thus,
@kbd{2 d} displays all the entries for the selected date and for the
following day.
Another way to display the diary entries for a date is to click
@kbd{Mouse-2} on the date, and then choose @kbd{Diary} from the menu
that appears.
@kindex m @r{(Calendar mode)}
@findex mark-diary-entries
To get a broader view of which days are mentioned in the diary, use
the @kbd{m} command. This displays the dates that have diary entries
in a different face (or places a @samp{+} after these dates, if
display with multiple faces is not available). The command applies both
to the currently visible months and to other months that subsequently
become visible by scrolling. To turn marking off and erase the current
marks, type @kbd{u}, which also turns off holiday marks
(@pxref{Holidays}).
@kindex s @r{(Calendar mode)}
@findex show-all-diary-entries
To see the full diary file, rather than just some of the entries, use
the @kbd{s} command.
Display of selected diary entries uses the selective display feature
to hide entries that don't apply.
The diary buffer as you see it is an illusion, so simply printing the
buffer does not print what you see on your screen. There is a special
command to print hard copy of the diary buffer @emph{as it appears};
this command is @kbd{M-x print-diary-entries}. It sends the data
directly to the printer. You can customize it like @code{lpr-region}
(@pxref{Hardcopy}).
@findex diary
The command @kbd{M-x diary} displays the diary entries for the current
date, independently of the calendar display, and optionally for the next
few days as well; the variable @code{number-of-diary-entries} specifies
how many days to include. @xref{Calendar, Customizing the Calendar
and Diary,, elisp, The Emacs Lisp Reference Manual}.
If you put @code{(diary)} in your @file{.emacs} file, this
automatically displays a window with the day's diary entries, when you
enter Emacs. The mode line of the displayed window shows the date and
any holidays that fall on that date.
@findex diary-mail-entries
@vindex diary-mail-days
Many users like to receive notice of events in their diary as email.
To send such mail to yourself, use the command @kbd{M-x
diary-mail-entries}. A prefix argument specifies how many days
(starting with today) to check; otherwise, the variable
@code{diary-mail-days} says how many days.
@node Format of Diary File
@subsection The Diary File
@cindex diary file
@vindex diary-file
Your @dfn{diary file} is a file that records events associated with
particular dates. The name of the diary file is specified by the
variable @code{diary-file}; @file{~/diary} is the default. The
@code{calendar} utility program supports a subset of the format allowed
by the Emacs diary facilities, so you can use that utility to view the
diary file, with reasonable results aside from the entries it cannot
understand.
Each entry in the diary file describes one event and consists of one
or more lines. An entry always begins with a date specification at the
left margin. The rest of the entry is simply text to describe the
event. If the entry has more than one line, then the lines after the
first must begin with whitespace to indicate they continue a previous
entry. Lines that do not begin with valid dates and do not continue a
preceding entry are ignored.
You can inhibit the marking of certain diary entries in the calendar
window; to do this, insert an ampersand (@samp{&}) at the beginning of
the entry, before the date. This has no effect on display of the entry
in the diary window; it affects only marks on dates in the calendar
window. Nonmarking entries are especially useful for generic entries
that would otherwise mark many different dates.
If the first line of a diary entry consists only of the date or day
name with no following blanks or punctuation, then the diary window
display doesn't include that line; only the continuation lines appear.
For example, this entry:
@example
02/11/1989
Bill B. visits Princeton today
2pm Cognitive Studies Committee meeting
2:30-5:30 Liz at Lawrenceville
4:00pm Dentist appt
7:30pm Dinner at George's
8:00-10:00pm concert
@end example
@noindent
appears in the diary window without the date line at the beginning.
This style of entry looks neater when you display just a single day's
entries, but can cause confusion if you ask for more than one day's
entries.
You can edit the diary entries as they appear in the window, but it is
important to remember that the buffer displayed contains the @emph{entire}
diary file, with portions of it concealed from view. This means, for
instance, that the @kbd{C-f} (@code{forward-char}) command can put point
at what appears to be the end of the line, but what is in reality the
middle of some concealed line.
@emph{Be careful when editing the diary entries!} Inserting
additional lines or adding/deleting characters in the middle of a
visible line cannot cause problems, but editing at the end of a line may
not do what you expect. Deleting a line may delete other invisible
entries that follow it. Before editing the diary, it is best to display
the entire file with @kbd{s} (@code{show-all-diary-entries}).
@node Date Formats
@subsection Date Formats
Here are some sample diary entries, illustrating different ways of
formatting a date. The examples all show dates in American order
(month, day, year), but Calendar mode supports European order (day,
month, year) as an option.
@example
4/20/93 Switch-over to new tabulation system
apr. 25 Start tabulating annual results
4/30 Results for April are due
*/25 Monthly cycle finishes
Friday Don't leave without backing up files
@end example
The first entry appears only once, on April 20, 1993. The second and
third appear every year on the specified dates, and the fourth uses a
wildcard (asterisk) for the month, so it appears on the 25th of every
month. The final entry appears every week on Friday.
You can use just numbers to express a date, as in
@samp{@var{month}/@var{day}} or @samp{@var{month}/@var{day}/@var{year}}.
This must be followed by a nondigit. In the date itself, @var{month}
and @var{day} are numbers of one or two digits. The optional @var{year}
is also a number, and may be abbreviated to the last two digits; that
is, you can use @samp{11/12/1989} or @samp{11/12/89}.
Dates can also have the form @samp{@var{monthname} @var{day}} or
@samp{@var{monthname} @var{day}, @var{year}}, where the month's name can
be spelled in full or abbreviated to three characters (with or without a
period). Case is not significant.
A date may be @dfn{generic}; that is, partially unspecified. Then the
entry applies to all dates that match the specification. If the date
does not contain a year, it is generic and applies to any year.
Alternatively, @var{month}, @var{day}, or @var{year} can be a @samp{*};
this matches any month, day, or year, respectively. Thus, a diary entry
@samp{3/*/*} matches any day in March of any year; so does @samp{march
*}.
@vindex european-calendar-style
@findex european-calendar
@findex american-calendar
If you prefer the European style of writing dates---in which the day
comes before the month---type @kbd{M-x european-calendar} while in the
calendar, or set the variable @code{european-calendar-style} to @code{t}
@emph{before} using any calendar or diary command. This mode interprets
all dates in the diary in the European manner, and also uses European
style for displaying diary dates. (Note that there is no comma after
the @var{monthname} in the European style.) To go back to the (default)
American style of writing dates, type @kbd{M-x american-calendar}.
You can use the name of a day of the week as a generic date which
applies to any date falling on that day of the week. You can abbreviate
the day of the week to three letters (with or without a period) or spell
it in full; case is not significant.
@node Adding to Diary
@subsection Commands to Add to the Diary
While in the calendar, there are several commands to create diary
entries:
@table @kbd
@item i d
Add a diary entry for the selected date (@code{insert-diary-entry}).
@item i w
Add a diary entry for the selected day of the week (@code{insert-weekly-diary-entry}).
@item i m
Add a diary entry for the selected day of the month (@code{insert-monthly-diary-entry}).
@item i y
Add a diary entry for the selected day of the year (@code{insert-yearly-diary-entry}).
@end table
@kindex i d @r{(Calendar mode)}
@findex insert-diary-entry
You can make a diary entry for a specific date by selecting that date
in the calendar window and typing the @kbd{i d} command. This command
displays the end of your diary file in another window and inserts the
date; you can then type the rest of the diary entry.
@kindex i w @r{(Calendar mode)}
@findex insert-weekly-diary-entry
@kindex i m @r{(Calendar mode)}
@findex insert-monthly-diary-entry
@kindex i y @r{(Calendar mode)}
@findex insert-yearly-diary-entry
If you want to make a diary entry that applies to a specific day of
the week, select that day of the week (any occurrence will do) and type
@kbd{i w}. This inserts the day-of-week as a generic date; you can then
type the rest of the diary entry. You can make a monthly diary entry in
the same fashion. Select the day of the month, use the @kbd{i m}
command, and type rest of the entry. Similarly, you can insert a yearly
diary entry with the @kbd{i y} command.
All of the above commands make marking diary entries by default. To
make a nonmarking diary entry, give a numeric argument to the command.
For example, @kbd{C-u i w} makes a nonmarking weekly diary entry.
When you modify the diary file, be sure to save the file before
exiting Emacs.
@node Special Diary Entries
@subsection Special Diary Entries
In addition to entries based on calendar dates, the diary file can
contain @dfn{sexp entries} for regular events such as anniversaries.
These entries are based on Lisp expressions (sexps) that Emacs evaluates
as it scans the diary file. Instead of a date, a sexp entry contains
@samp{%%} followed by a Lisp expression which must begin and end with
parentheses. The Lisp expression determines which dates the entry
applies to.
Calendar mode provides commands to insert certain commonly used
sexp entries:
@table @kbd
@item i a
Add an anniversary diary entry for the selected date
(@code{insert-anniversary-diary-entry}).
@item i b
Add a block diary entry for the current region
(@code{insert-block-diary-entry}).
@item i c
Add a cyclic diary entry starting at the date
(@code{insert-cyclic-diary-entry}).
@end table
@kindex i a @r{(Calendar mode)}
@findex insert-anniversary-diary-entry
If you want to make a diary entry that applies to the anniversary of a
specific date, move point to that date and use the @kbd{i a} command.
This displays the end of your diary file in another window and inserts
the anniversary description; you can then type the rest of the diary
entry. The entry looks like this:
@findex diary-anniversary
@example
%%(diary-anniversary 10 31 1948) Arthur's birthday
@end example
@noindent
This entry applies to October 31 in any year after 1948; @samp{10 31
1948} specifies the date. (If you are using the European calendar
style, the month and day are interchanged.) The reason this expression
requires a beginning year is that advanced diary functions can use it to
calculate the number of elapsed years.
A @dfn{block} diary entry applies to a specified range of consecutive
dates. Here is a block diary entry that applies to all dates from June
24, 1990 through July 10, 1990:
@findex diary-block
@example
%%(diary-block 6 24 1990 7 10 1990) Vacation
@end example
@noindent
The @samp{6 24 1990} indicates the starting date and the @samp{7 10 1990}
indicates the stopping date. (Again, if you are using the European calendar
style, the month and day are interchanged.)
@kindex i b @r{(Calendar mode)}
@findex insert-block-diary-entry
To insert a block entry, place point and the mark on the two
dates that begin and end the range, and type @kbd{i b}. This command
displays the end of your diary file in another window and inserts the
block description; you can then type the diary entry.
@kindex i c @r{(Calendar mode)}
@findex insert-cyclic-diary-entry
@dfn{Cyclic} diary entries repeat after a fixed interval of days. To
create one, select the starting date and use the @kbd{i c} command. The
command prompts for the length of interval, then inserts the entry,
which looks like this:
@findex diary-cyclic
@example
%%(diary-cyclic 50 3 1 1990) Renew medication
@end example
@noindent
This entry applies to March 1, 1990 and every 50th day following;
@samp{3 1 1990} specifies the starting date. (If you are using the
European calendar style, the month and day are interchanged.)
All three of these commands make marking diary entries. To insert a
nonmarking entry, give a numeric argument to the command. For example,
@kbd{C-u i a} makes a nonmarking anniversary diary entry.
Marking sexp diary entries in the calendar is @emph{extremely}
time-consuming, since every date visible in the calendar window must be
individually checked. So it's a good idea to make sexp diary entries
nonmarking (with @samp{&}) when possible.
Another sophisticated kind of sexp entry, a @dfn{floating} diary entry,
specifies a regularly occurring event by offsets specified in days,
weeks, and months. It is comparable to a crontab entry interpreted by
the @code{cron} utility. Here is a nonmarking, floating diary entry
that applies to the last Thursday in November:
@findex diary-float
@example
&%%(diary-float 11 4 -1) American Thanksgiving
@end example
@noindent
The 11 specifies November (the eleventh month), the 4 specifies Thursday
(the fourth day of the week, where Sunday is numbered zero), and the
@minus{}1 specifies ``last'' (1 would mean ``first,'' 2 would mean
``second,'' @minus{}2 would mean ``second-to-last,'' and so on). The
month can be a single month or a list of months. Thus you could change
the 11 above to @samp{'(1 2 3)} and have the entry apply to the last
Thursday of January, February, and March. If the month is @code{t}, the
entry applies to all months of the year.@refill
Most generally, sexp diary entries can perform arbitrary
computations to determine when they apply. @xref{Sexp Diary Entries,,
Sexp Diary Entries, elisp, The Emacs Lisp Reference Manual}.
@node Appointments
@section Appointments
@cindex appointment notification
If you have a diary entry for an appointment, and that diary entry
begins with a recognizable time of day, Emacs can warn you, several
minutes beforehand, that that appointment is pending. Emacs alerts you
to the appointment by displaying a message in the mode line.
@vindex diary-hook
@findex appt-make-list
To enable appointment notification, you must enable the time display
feature of Emacs, @kbd{M-x display-time} (@pxref{Mode Line}). You must
also add the function @code{appt-make-list} to the
@code{diary-hook}, like this:
@example
(add-hook 'diary-hook 'appt-make-list)
@end example
@noindent
Adding this text to your @file{.emacs} file does the whole job:
@example
(display-time)
(add-hook 'diary-hook 'appt-make-list)
(diary 0)
@end example
With these preparations done, when you display the diary (either with
the @kbd{d} command in the calendar window or with the @kbd{M-x diary}
command), it sets up an appointment list of all the diary entries found
with recognizable times of day, and reminds you just before each of
them.
For example, suppose the diary file contains these lines:
@example
Monday
9:30am Coffee break
12:00pm Lunch
@end example
@noindent
Then on Mondays, after you have displayed the diary, you will be
reminded at 9:20am about your coffee break and at 11:50am about lunch.
You can write times in am/pm style (with @samp{12:00am} standing
for midnight and @samp{12:00pm} standing for noon), or 24-hour
European/military style. You need not be consistent; your diary file
can have a mixture of the two styles.
@vindex appt-display-diary
Emacs updates the appointments list automatically just after
midnight. This also displays the next day's diary entries in the diary
buffer, unless you set @code{appt-display-diary} to @code{nil}.
@findex appt-add
@findex appt-delete
@cindex alarm clock
You can also use the appointment notification facility like an alarm
clock. The command @kbd{M-x appt-add} adds entries to the appointment
list without affecting your diary file. You delete entries from the
appointment list with @kbd{M-x appt-delete}.
@vindex appt-issue-message
You can turn off the appointment notification feature at any time by
setting @code{appt-issue-message} to @code{nil}.
@node Daylight Savings
@section Daylight Savings Time
@cindex daylight savings time
Emacs understands the difference between standard time and daylight
savings time---the times given for sunrise, sunset, solstices,
equinoxes, and the phases of the moon take that into account. The rules
for daylight savings time vary from place to place and have also varied
historically from year to year. To do the job properly, Emacs needs to
know which rules to use.
@vindex calendar-daylight-savings-starts
@vindex calendar-daylight-savings-ends
Some operating systems keep track of the rules that apply to the place
where you are; on these systems, Emacs gets the information it needs
from the system automatically. If some or all of this information is
missing, Emacs fills in the gaps with the rules currently used in
Cambridge, Massachusetts. If the resulting rules are not what you want,
you can tell Emacs the rules to use by setting certain variables:
@code{calendar-daylight-savings-starts} and
@code{calendar-daylight-savings-ends}.
These values should be Lisp expressions that refer to the variable
@code{year}, and evaluate to the Gregorian date on which daylight
savings time starts or (respectively) ends, in the form of a list
@code{(@var{month} @var{day} @var{year})}. The values should be
@code{nil} if your area does not use daylight savings time.
Emacs uses these expressions to determine the starting date of
daylight savings time for the holiday list and for correcting times of
day in the solar and lunar calculations.
The values for Cambridge, Massachusetts are as follows:
@example
(calendar-nth-named-day 1 0 4 year)
(calendar-nth-named-day -1 0 10 year)
@end example
@noindent
That is, the first 0th day (Sunday) of the fourth month (April) in
the year specified by @code{year}, and the last Sunday of the tenth month
(October) of that year. If daylight savings time were
changed to start on October 1, you would set
@code{calendar-daylight-savings-starts} to this:
@example
(list 10 1 year)
@end example
If there is no daylight savings time at your location, or if you want
all times in standard time, set @code{calendar-daylight-savings-starts}
and @code{calendar-daylight-savings-ends} to @code{nil}.
@vindex calendar-daylight-time-offset
The variable @code{calendar-daylight-time-offset} specifies the
difference between daylight savings time and standard time, measured in
minutes. The value for Cambridge, Massachusetts is 60.
@c @vindex calendar-daylight-savings-starts-time too long!
@vindex calendar-daylight-savings-ends-time
The two variables @code{calendar-daylight-savings-starts-time} and
@code{calendar-daylight-savings-ends-time} specify the number of minutes
after midnight local time when the transition to and from daylight
savings time should occur. For Cambridge, Massachusetts both variables'
values are 120.