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
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-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-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.
This chapter contains several brief topics that do not fit anywhere else: reading netnews, running shell commands and shell subprocesses, using a single shared Emacs for utilities that expect to run an editor as a subprocess, printing hardcopy, sorting text, narrowing display to part of the buffer, editing double-column files and binary files, saving an Emacs session for later resumption, emulating other editors, various diversions and amusements.