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Do not assume DST starts/ends on the same date in every year.
(calendar-dst-check-each-year-flag): New customizable variable. (calendar-dst-find-data): New function, extracted from calendar-current-time-zone. (calendar-current-time-zone): Use calendar-dst-find-data. (calendar-dst-transition-cache): New variable. (calendar-dst-find-startend, calendar-dst-starts) (calendar-dst-ends): New functions. (calendar-daylight-savings-starts) (calendar-daylight-savings-ends): Change value to use calendar-dst-starts, calendar-dst-ends; respectively.
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Glenn Morris
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86f46920ff
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cd72c39915
@ -42,6 +42,16 @@
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(require 'calendar)
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(require 'cal-persia)
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(defcustom calendar-dst-check-each-year-flag t
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"Non-nil means to check each year for DST transitions as needed.
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nil means to assume the next two transitions found after the
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current date apply to all years. This is faster, but not always
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correct, since the dates of Daylight Saving transitions sometimes
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change."
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:type 'boolean
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:version "22.1"
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:group 'calendar)
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(defvar calendar-current-time-zone-cache nil
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"Cache for result of calendar-current-time-zone.")
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@ -199,6 +209,74 @@ The result has the proper form for calendar-daylight-savings-starts'."
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(cdr candidate-rules)))
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(car candidate-rules)))
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;; TODO it might be better to extract this information directly from
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;; the system timezone database. But cross-platform...?
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;; See thread
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;; http://lists.gnu.org/archive/html/emacs-pretest-bug/2006-11/msg00060.html
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(defun calendar-dst-find-data (&optional time)
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"Find data on the first Daylight Saving Time transitions after TIME.
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TIME defaults to `current-time'. Return value is as described
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for `calendar-current-time-zone'."
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(let* ((t0 (or time (current-time)))
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(t0-zone (current-time-zone t0))
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(t0-utc-diff (car t0-zone))
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(t0-name (car (cdr t0-zone))))
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(if (not t0-utc-diff)
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;; Little or no time zone information is available.
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(list nil nil t0-name t0-name nil nil nil nil)
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(let* ((t1 (calendar-next-time-zone-transition t0))
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(t2 (and t1 (calendar-next-time-zone-transition t1))))
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(if (not t2)
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;; This locale does not have daylight savings time.
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(list (/ t0-utc-diff 60) 0 t0-name t0-name nil nil 0 0)
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;; Use heuristics to find daylight savings parameters.
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(let* ((t1-zone (current-time-zone t1))
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(t1-utc-diff (car t1-zone))
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(t1-name (car (cdr t1-zone)))
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(t1-date-sec (calendar-absolute-from-time t1 t0-utc-diff))
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(t2-date-sec (calendar-absolute-from-time t2 t1-utc-diff))
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;; TODO When calendar-dst-check-each-year-flag is non-nil,
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;; the rules can be simpler than they currently are.
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(t1-rules (calendar-time-zone-daylight-rules
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(car t1-date-sec) t0-utc-diff))
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(t2-rules (calendar-time-zone-daylight-rules
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(car t2-date-sec) t1-utc-diff))
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(t1-time (/ (cdr t1-date-sec) 60))
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(t2-time (/ (cdr t2-date-sec) 60)))
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(cons
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(/ (min t0-utc-diff t1-utc-diff) 60)
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(cons
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(/ (abs (- t0-utc-diff t1-utc-diff)) 60)
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(if (< t0-utc-diff t1-utc-diff)
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(list t0-name t1-name t1-rules t2-rules t1-time t2-time)
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(list t1-name t0-name t2-rules t1-rules t2-time t1-time)
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)))))))))
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(defvar calendar-dst-transition-cache nil
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"Internal cal-dst variable storing date of Daylight Saving Time transitions.
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Value is a list with elements of the form (YEAR START END), where
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START and END are expressions that when evaluated return the
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start and end dates (respectively) for DST in YEAR. Used by the
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function `calendar-dst-find-startend'.")
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(defun calendar-dst-find-startend (year)
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"Find the dates in YEAR on which Daylight Saving Time starts and ends.
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Returns a list (YEAR START END), where START and END are
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expressions that when evaluated return the start and end dates,
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respectively. This function first attempts to use pre-calculated
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data from `calendar-dst-transition-cache', otherwise it calls
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`calendar-dst-find-data' (and adds the results to the cache)."
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(let ((e (assoc year calendar-dst-transition-cache))
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f)
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(or e
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(progn
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(setq e (calendar-dst-find-data (encode-time 1 0 0 1 1 year))
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f (nth 4 e)
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e (list year f (nth 5 e))
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calendar-dst-transition-cache
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(append calendar-dst-transition-cache (list e)))
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e))))
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(defun calendar-current-time-zone ()
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"Return UTC difference, dst offset, names and rules for current time zone.
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@ -226,42 +304,8 @@ DST-ZONE are equal, and all the DST-* integer variables are 0.
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Some operating systems cannot provide all this information to Emacs; in this
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case, `calendar-current-time-zone' returns a list containing nil for the data
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it can't find."
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(or
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calendar-current-time-zone-cache
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(setq
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calendar-current-time-zone-cache
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(let* ((t0 (current-time))
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(t0-zone (current-time-zone t0))
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(t0-utc-diff (car t0-zone))
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(t0-name (car (cdr t0-zone))))
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(if (not t0-utc-diff)
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;; Little or no time zone information is available.
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(list nil nil t0-name t0-name nil nil nil nil)
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(let* ((t1 (calendar-next-time-zone-transition t0))
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(t2 (and t1 (calendar-next-time-zone-transition t1))))
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(if (not t2)
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;; This locale does not have daylight savings time.
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(list (/ t0-utc-diff 60) 0 t0-name t0-name nil nil 0 0)
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;; Use heuristics to find daylight savings parameters.
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(let* ((t1-zone (current-time-zone t1))
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(t1-utc-diff (car t1-zone))
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(t1-name (car (cdr t1-zone)))
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(t1-date-sec (calendar-absolute-from-time t1 t0-utc-diff))
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(t2-date-sec (calendar-absolute-from-time t2 t1-utc-diff))
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(t1-rules (calendar-time-zone-daylight-rules
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(car t1-date-sec) t0-utc-diff))
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(t2-rules (calendar-time-zone-daylight-rules
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(car t2-date-sec) t1-utc-diff))
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(t1-time (/ (cdr t1-date-sec) 60))
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(t2-time (/ (cdr t2-date-sec) 60)))
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(cons
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(/ (min t0-utc-diff t1-utc-diff) 60)
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(cons
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(/ (abs (- t0-utc-diff t1-utc-diff)) 60)
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(if (< t0-utc-diff t1-utc-diff)
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(list t0-name t1-name t1-rules t2-rules t1-time t2-time)
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(list t1-name t0-name t2-rules t1-rules t2-time t1-time)
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)))))))))))
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(unless calendar-current-time-zone-cache
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(setq calendar-current-time-zone-cache (calendar-dst-find-data))))
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;;; The following eight defvars relating to daylight savings time should NOT be
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;;; marked to go into loaddefs.el where they would be evaluated when Emacs is
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@ -293,12 +337,32 @@ For example, \"EST\" in New York City, \"PST\" for Los Angeles.")
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"*Abbreviated name of daylight-savings time zone at `calendar-location-name'.
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For example, \"EDT\" in New York City, \"PDT\" for Los Angeles.")
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(defun calendar-dst-starts (year)
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"Return the date of YEAR on which Daylight Saving Time starts.
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This function respects the value of `calendar-dst-check-each-year-flag'."
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(or (let ((expr (if calendar-dst-check-each-year-flag
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(cadr (calendar-dst-find-startend year))
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(nth 4 calendar-current-time-zone-cache))))
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(if expr (eval expr)))
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(and (not (zerop calendar-daylight-time-offset))
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(calendar-nth-named-day 1 0 4 year))))
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(defun calendar-dst-ends (year)
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"Return the date of YEAR on which Daylight Saving Time ends.
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This function respects the value of `calendar-dst-check-each-year-flag'."
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(or (let ((expr (if calendar-dst-check-each-year-flag
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(nth 2 (calendar-dst-find-startend year))
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(nth 5 calendar-current-time-zone-cache))))
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(if expr (eval expr)))
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(and (not (zerop calendar-daylight-time-offset))
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(calendar-nth-named-day -1 0 10 year))))
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;;;###autoload
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(put 'calendar-daylight-savings-starts 'risky-local-variable t)
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(defvar calendar-daylight-savings-starts
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(or (car (nthcdr 4 calendar-current-time-zone-cache))
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(and (not (zerop calendar-daylight-time-offset))
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'(calendar-nth-named-day 1 0 4 year)))
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'(calendar-dst-starts year)
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"*Sexp giving the date on which daylight savings time starts.
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This is an expression in the variable `year' whose value gives the Gregorian
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date in the form (month day year) on which daylight savings time starts. It is
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@ -319,9 +383,7 @@ If the locale never uses daylight savings time, set this to nil.")
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;;;###autoload
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(put 'calendar-daylight-savings-ends 'risky-local-variable t)
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(defvar calendar-daylight-savings-ends
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(or (car (nthcdr 5 calendar-current-time-zone-cache))
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(and (not (zerop calendar-daylight-time-offset))
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'(calendar-nth-named-day -1 0 10 year)))
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'(calendar-dst-ends year)
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"*Sexp giving the date on which daylight savings time ends.
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This is an expression in the variable `year' whose value gives the Gregorian
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date in the form (month day year) on which daylight savings time ends. It is
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