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(emacs)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 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.