39.13 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.
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:
calendar-daylight-savings-starts and
calendar-daylight-savings-ends.
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, Massachusetts both variables'
values are 120.
