(lemacs.info)Daylight Savings


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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 default choice of
rules is not appropriate for your location, you can tell Emacs the
rules to use by setting certain variables.

   These variables are `calendar-daylight-savings-starts' and
`calendar-daylight-savings-ends'.  Their 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)

i.e. 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)

   For a more complex example, suppose daylight savings time begins on
the first of Nisan on the Hebrew calendar.  You would set
`calendar-daylight-savings-starts' as follows:

     (calendar-gregorian-from-absolute
       (calendar-absolute-from-hebrew
         (list 1 1 (+ year 3760))))

because Nisan is the first month in the Hebrew calendar and the Hebrew
year differs from the Gregorian year by 3760 at Nisan.

   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'.

   This variable specifies the difference between daylight savings time
and standard time, measured in minutes.  The value for Cambridge is 60.

   These variables specify is 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.


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