Emacs provides the functions of a desk calendar, with a diary of planned or past events. To enter the calendar, type 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 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.
To exit the calendar, type q. Mouse-3 in the calendar brings up a menu of commonly used calendar features. See section `Calendar' in The Emacs Lisp Reference Manual, for customization information about the calendar and diary.
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, convert it to other calendars; moving longer time periods is also useful simply to scroll the calendar.
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.
calendar-forward-day).
calendar-backward-day).
calendar-forward-week).
calendar-backward-week).
calendar-forward-month).
calendar-backward-month).
calendar-forward-year).
calendar-forward-year).
The day and week commands are natural analogues of the usual Emacs commands for moving by characters and by lines. Just as 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 C-p moves to the same day in the previous week.
The arrow keys are equivalent to C-f, C-b, C-n and C-p, just as they normally are in other modes.
The commands for motion by months and years work like those for weeks, but move a larger distance. The month commands M-} and M-{ move forward or backward by an entire month's time. The year commands C-x ] and 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, 100 C-f moves point 100 days forward from its present location.
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:
calendar-beginning-of-week).
calendar-end-of-week).
calendar-beginning-of-month).
calendar-end-of-month).
calendar-beginning-of-year).
calendar-end-of-year).
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.
By default, weeks begin on Sunday. To make them begin on Monday
instead, set the variable calendar-week-start-day to 1.
Calendar mode provides commands for moving to a particular date specified in various ways.
calendar-goto-date).
calendar-other-month).
calendar-goto-today).
g d (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 `1990', not `90'.
o (calendar-other-month) prompts for a month and year,
then centers the three-month calendar around that month.
You can return to today's date with .
(calendar-goto-today).
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.
scroll-calendar-left).
scroll-calendar-right).
scroll-calendar-left-three-months).
scroll-calendar-right-three-months).
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. C-x < scrolls the calendar contents one month to the left; that is, it moves the display forward in time. C-x > scrolls the contents to the right, which moves backwards in time.
The commands C-v and M-v scroll the calendar by an entire "screenful"---three months--in analogy with the usual meaning of these commands. C-v makes later dates visible and M-v makes earlier dates visible. These commands take a numeric argument as a repeat count; in particular, since C-u multiplies the next command by four, typing C-u C-v scrolls the calendar forward by a year and typing C-u M-v scrolls the calendar backward by a year.
The function keys NEXT and PRIOR are equivalent to C-v and M-v, just as they are in other modes.
calendar-count-days-region).
To determine the number of days in the region, type M-=
(calendar-count-days-region). The numbers of days printed is
inclusive; that is, it includes the days specified by mark and
point.
calendar-print-day-of-year).
redraw-calendar).
scroll-other-window).
exit-calendar).
To print the number of days elapsed since the start of the year, or
the number of days remaining in the year, type the p d command
(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.
If the calendar window text gets corrupted, type C-c C-l
(redraw-calendar) to redraw it. (This can only happen if you use
non-Calendar-mode editing commands.)
In Calendar mode, you can use SPC (scroll-other-window)
to scroll the other window. This is handy when you display a list of
holidays or diary entries in another window.
To exit from the calendar, type q (exit-calendar). This
buries all buffers related to the calendar and returns the window display
to what it was when you entered the calendar.
The Emacs calendar knows about all major and many minor holidays, and can display them.
calendar-cursor-holidays).
mark-calendar-holidays).
calendar-unmark).
list-calendar-holidays).
To see if any holidays fall on a given date, position point on that date in the calendar window and use the h command. Alternatively, click on that date with Mouse-2 and then choose 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.
To view the distribution of holidays for all the dates shown in the calendar, use the x command. This displays the dates that are holidays in a different face (or places a `*' 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 u, which also erases any diary marks (see section The Diary).
To get even more detailed information, use the a command, which displays a separate buffer containing a list of all holidays in the current three-month range. You can use SPC in the calendar window to scroll that list.
The command 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 C-u M-x holidays, which prompts for the month and year.
The holidays known to Emacs include American holidays and the major Christian, Jewish, and Islamic holidays; also the solstices and equinoxes.
The dates used by Emacs for holidays are based on 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 American law mandates that daylight savings time begins on the first Sunday in April. In an American locale, Emacs always uses this definition, even though it is wrong for some prior years.
Special calendar commands can tell you, to within a minute or two, the times of sunrise and sunset for any date.
calendar-sunrise-sunset).
Within the calendar, to display the local times of sunrise and sunset in the echo area, move point to the date you want, and type S. Alternatively, click Mouse-2 on the date, then choose Sunrise/Sunset from the menu that appears. The command 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 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 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:
(setq calendar-latitude 40.1) (setq calendar-longitude -88.2) (setq calendar-location-name "Urbana, IL")
Use one decimal place in the values of calendar-latitude and
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:
(setq calendar-time-zone -360) (setq calendar-standard-time-zone-name "CST") (setq calendar-daylight-time-zone-name "CDT")
The value of calendar-time-zone is the number of minutes
difference between your local standard time and Coordinated Universal
Time (Greenwich time). The values of
calendar-standard-time-zone-name and
calendar-daylight-time-zone-name are the abbreviations used in
your time zone. Emacs displays the times of sunrise and sunset
corrected for daylight savings time. See section Daylight Savings Time,
for how daylight savings time is determined.
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."
calendar-phases-of-moon).
Within the calendar, use the 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.
Outside the calendar, use the command 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 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 calendar-time-zone is void, Coordinated Universal
Time (the Greenwich time zone) is used. See section Daylight Savings Time.
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. 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.
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 following commands describe the selected date (the date at point) in various other calendar systems:
calendar-print-iso-date).
calendar-print-julian-date).
calendar-print-astro-day-number).
calendar-print-hebrew-date).
calendar-print-islamic-date).
calendar-print-french-date).
calendar-print-mayan-date).
If you are using X windows, the easiest way to translate a date into other calendars is to click on it with Mouse-2, then choose 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 p is a mnemonic for "print" since Emacs "prints" the equivalent date in the echo area.
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.
calendar-goto-iso-date).
calendar-goto-julian-date).
calendar-goto-astro-day-number).
calendar-goto-hebrew-date).
calendar-goto-islamic-date).
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 (see section 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.
Here are the commands to select dates based on the Mayan calendar:
calendar-goto-mayan-long-count-date).
calendar-next-tzolkin-date).
calendar-previous-tzolkin-date).
calendar-next-haab-date).
calendar-previous-haab-date).
calendar-next-calendar-round-date).
calendar-previous-calendar-round-date).
To understand these commands, you need to understand the Mayan calendars. The long count is a counting of days with these 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. Since this cycle repeats endlessly, Emacs provides commands to move backward and forward to the previous or next point in the cycle. Type 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 g m n t to go to the next occurrence of a tzolkin date.
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 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 occurrence of a 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 of a combination. These commands signal an error if the haab/tzolkin date combination you have typed is impossible.
Emacs uses strict completion (see section Strict Completion) whenever it asks you to type a Mayan name, so you don't have to worry about spelling.
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 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 `~/diary' as the diary file. This is the
same file that the calendar utility uses. A sample
`~/diary' file is:
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.
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.
Once you have created a `~/diary' file, you can use the calendar to view it. You can also view today's events outside of Calendar mode.
view-diary-entries).
show-all-diary-entries).
mark-diary-entries).
calendar-unmark).
Displaying the diary entries with 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 d, it shows all the diary entries for that many successive days. Thus, 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 Mouse-2 on the date, and then choose Diary from the menu that appears.
To get a broader view of which days are mentioned in the diary, use the m command. This displays the dates that have diary entries fall in a different face (or places a `+' 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 u, which also turns off holiday marks (see section Holidays).
To see the full diary file, rather than just some of the entries, use the s command.
Display of selected diary entries uses the selective display feature to hide entries that don't apply. This is the same feature that Outline mode uses to show part of an outline (see section Outline Mode).
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 as it appears;
this command is M-x print-diary-entries. It sends the data
directly to the printer. You can customize it like lpr-region
(see section Hardcopy Output).
The command 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 number-of-diary-entries specifies
how many days to include. See section `Calendar/Diary Options' in The Emacs Lisp Reference Manual.
If you put (diary) in your `.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.
Your diary file is a file that records events associated with
particular dates. The name of the diary file is specified by the
variable diary-file; `~/diary' is the default. The
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 (`&') 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:
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
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 entire
diary file, with portions of it concealed from view. This means, for
instance, that the C-f (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.
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 s (show-all-diary-entries).
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.
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
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 `month/day' or `month/day/year'. This must be followed by a nondigit. In the date itself, month and day are numbers of one or two digits. The optional year is also a number, and may be abbreviated to the last two digits; that is, you can use `11/12/1989' or `11/12/89'.
Dates can also have the form `monthname day' or `monthname day, 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 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, month, day, or year can be a `*'; this matches any month, day, or year, respectively. Thus, a diary entry `3/*/*' matches any day in March of any year; so does `march *'.
If you prefer the European style of writing dates--in which the day
comes before the month--type M-x european-calendar while in the
calendar, or set the variable european-calendar-style to t
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 monthname in the European style.) To go back to the (default)
American style of writing dates, type 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.
While in the calendar, there are several commands to create diary entries:
insert-diary-entry).
insert-weekly-diary-entry).
insert-monthly-diary-entry).
insert-yearly-diary-entry).
You can make a diary entry for a specific date by selecting that date in the calendar window and typing the 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.
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 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 i m command, and type rest of the entry. Similarly, you can insert a yearly diary entry with the 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, 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.
In addition to entries based on calendar dates, the diary file can contain special 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 special entry contains `%%' 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 special entries:
insert-anniversary-diary-entry).
insert-block-diary-entry).
insert-cyclic-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 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:
%%(diary-anniversary 10 31 1948) Arthur's birthday
This entry applies to October 31 in any year after 1948; `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 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:
%%(diary-block 6 24 1990 7 10 1990) Vacation
The `6 24 1990' indicates the starting date and the `7 10 1990' indicates the stopping date. (Again, if you are using the European calendar style, the month and day are interchanged.)
To insert a block entry, place point and the mark on the two dates that begin and end the range, and type 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.
Cyclic diary entries repeat after a fixed interval of days. To create one, select the starting date and use the i c command. The command prompts for the length of interval, then inserts the entry, which looks like this:
%%(diary-cyclic 50 3 1 1990) Renew medication
This entry applies to March 1, 1990 and every 50th day following; `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, C-u i a makes a nonmarking anniversary diary entry.
Marking sexp diary entries in the calendar is 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 `&') when possible.
Another sophisticated kind of sexp entry, a 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 cron utility. Here is a nonmarking, floating diary entry
that applies to the last Thursday in November:
&%%(diary-float 11 4 -1) American Thanksgiving
The 11 specifies November (the eleventh month), the 4 specifies Thursday
(the fourth day of the week, where Sunday is numbered zero), and the
-1 specifies "last" (1 would mean "first", 2 would mean
"second", -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 `'(1 2 3)' and have the entry apply to the last
Thursday of January, February, and March. If the month is t, the
entry applies to all months of the year.
Most generally, special diary entries can perform arbitrary computations to determine when they apply. See section `Sexp Diary Entries' in The Emacs Lisp Reference Manual.
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.
To enable appointment notification, you must enable the time display
feature of Emacs, M-x display-time (see section The Mode Line). You must
also add the function appt-make-list to the
diary-hook, like this:
(add-hook 'diary-hook 'appt-make-list)
With these preparations done, when you display the diary (either with the d command in the calendar window or with the 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:
Monday 9:30am Coffee break 12:00pm Lunch
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 conventional American am/pm style, or 24-hour European/military style. You need not be consistent; your diary file can have a mixture of the two styles.
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 appt-display-diary to nil.
You can also use the appointment notification facility like an alarm clock. The command M-x appt-add adds entries to the appointment list without affecting your diary file. You delete entries from the appointment list with M-x appt-delete.
You can turn off the appointment notification feature at any time by
setting appt-issue-message to nil.
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.
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.
These values should be Lisp
expressions that refer to the variable year, and evaluate to the
Gregorian date on which daylight savings time starts or (respectively)
ends, in the form of a list (month day year).
The values should be 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:
(calendar-nth-named-day 1 0 4 year) (calendar-nth-named-day -1 0 10 year)
That is, the first 0th day (Sunday) of the fourth month (April) in
the year specified by 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
calendar-daylight-savings-starts to this:
(list 10 1 year)
If there is no daylight savings time at your location, or if you want
all times in standard time, set calendar-daylight-savings-starts
and calendar-daylight-savings-ends to nil.
The variable calendar-daylight-time-offset specifies the
difference between daylight savings time and standard time, measured in
minutes. The value for Cambridge, Massachusetts is 60.
The two variables calendar-daylight-savings-starts-time and
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, both variables'
values are 120.