1 ;;; cal-dst.el --- calendar functions for daylight savings rules
3 ;; Copyright (C) 1993, 1994, 1995, 1996, 2001, 2002, 2003, 2004, 2005,
4 ;; 2006, 2007 Free Software Foundation, Inc.
6 ;; Author: Paul Eggert <eggert@twinsun.com>
7 ;; Edward M. Reingold <reingold@cs.uiuc.edu>
8 ;; Maintainer: Glenn Morris <rgm@gnu.org>
10 ;; Human-Keywords: daylight savings time, calendar, diary, holidays
12 ;; This file is part of GNU Emacs.
14 ;; GNU Emacs is free software; you can redistribute it and/or modify
15 ;; it under the terms of the GNU General Public License as published by
16 ;; the Free Software Foundation; either version 2, or (at your option)
19 ;; GNU Emacs is distributed in the hope that it will be useful,
20 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
21 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
22 ;; GNU General Public License for more details.
24 ;; You should have received a copy of the GNU General Public License
25 ;; along with GNU Emacs; see the file COPYING. If not, write to the
26 ;; Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
27 ;; Boston, MA 02110-1301, USA.
31 ;; This collection of functions implements the features of calendar.el and
32 ;; holiday.el that deal with daylight savings time.
34 ;; Comments, corrections, and improvements should be sent to
35 ;; Edward M. Reingold Department of Computer Science
36 ;; (217) 333-6733 University of Illinois at Urbana-Champaign
37 ;; reingold@cs.uiuc.edu 1304 West Springfield Avenue
38 ;; Urbana, Illinois 61801
45 (defcustom calendar-dst-check-each-year-flag t
46 "Non-nil means to check each year for DST transitions as needed.
47 Otherwise assume the next two transitions found after the
48 current date apply to all years. This is faster, but not always
49 correct, since the dates of Daylight Saving transitions sometimes
55 (defvar calendar-current-time-zone-cache nil
56 "Cache for result of `calendar-current-time-zone'.")
57 (put 'calendar-current-time-zone-cache
'risky-local-variable t
)
59 (defvar calendar-system-time-basis
60 (calendar-absolute-from-gregorian '(1 1 1970))
61 "Absolute date of starting date of system clock.")
63 (defun calendar-absolute-from-time (x utc-diff
)
64 "Absolute local date of time X; local time is UTC-DIFF seconds from UTC.
66 X is (HIGH . LOW) or (HIGH LOW . IGNORED) where HIGH and LOW are the
67 high and low 16 bits, respectively, of the number of seconds since
68 1970-01-01 00:00:00 UTC, ignoring leap seconds.
70 Returns the pair (ABS-DATE . SECONDS) where SECONDS after local midnight on
71 absolute date ABS-DATE is the equivalent moment to X."
74 (l (+ utc-diff
(if (numberp xtail
) xtail
(car xtail
))))
75 (u (+ (* 512 (mod h
675)) (floor l
128))))
76 ;; Overflow is a terrible thing!
77 (cons (+ calendar-system-time-basis
78 ;; floor((2^16 h +l) / (60*60*24))
79 (* 512 (floor h
675)) (floor u
675))
80 ;; (2^16 h +l) mod (60*60*24)
81 (+ (* (mod u
675) 128) (mod l
128)))))
83 (defun calendar-time-from-absolute (abs-date s
)
84 "Time of absolute date ABS-DATE, S seconds after midnight.
86 Returns the list (HIGH LOW) where HIGH and LOW are the high and low
87 16 bits, respectively, of the number of seconds 1970-01-01 00:00:00 UTC,
88 ignoring leap seconds, that is the equivalent moment to S seconds after
89 midnight UTC on absolute date ABS-DATE."
90 (let* ((a (- abs-date calendar-system-time-basis
))
91 (u (+ (* 163 (mod a
512)) (floor s
128))))
92 ;; Overflow is a terrible thing!
94 ;; floor((60*60*24*a + s) / 2^16)
95 (+ a
(* 163 (floor a
512)) (floor u
512))
96 ;; (60*60*24*a + s) mod 2^16
97 (+ (* 128 (mod u
512)) (mod s
128)))))
99 (defun calendar-next-time-zone-transition (time)
100 "Return the time of the next time zone transition after TIME.
101 Both TIME and the result are acceptable arguments to current-time-zone.
102 Return nil if no such transition can be found."
103 (let* ((base 65536);; 2^16 = base of current-time output
104 (quarter-multiple 120);; approx = (seconds per quarter year) / base
105 (time-zone (current-time-zone time
))
106 (time-utc-diff (car time-zone
))
109 (hi-utc-diff time-utc-diff
)
111 ;; Heuristic: probe the time zone offset in the next three calendar
112 ;; quarters, looking for a time zone offset different from TIME.
113 (while (and quarters
(eq time-utc-diff hi-utc-diff
))
114 (setq hi
(cons (+ (car time
) (* (car quarters
) quarter-multiple
)) 0))
115 (setq hi-zone
(current-time-zone hi
))
116 (setq hi-utc-diff
(car hi-zone
))
117 (setq quarters
(cdr quarters
)))
121 (not (eq time-utc-diff hi-utc-diff
))
122 ;; Now HI is after the next time zone transition.
123 ;; Set LO to TIME, and then binary search to increase LO and decrease HI
124 ;; until LO is just before and HI is just after the time zone transition.
125 (let* ((tail (cdr time
))
126 (lo (cons (car time
) (if (numberp tail
) tail
(car tail
))))
129 ;; Set PROBE to halfway between LO and HI, rounding down.
130 ;; If PROBE equals LO, we are done.
131 (let* ((lsum (+ (cdr lo
) (cdr hi
)))
132 (hsum (+ (car lo
) (car hi
) (/ lsum base
)))
133 (hsumodd (logand 1 hsum
)))
134 (setq probe
(cons (/ (- hsum hsumodd
) 2)
135 (/ (+ (* hsumodd base
) (% lsum base
)) 2)))
136 (not (equal lo probe
)))
137 ;; Set either LO or HI to PROBE, depending on probe results.
138 (if (eq (car (current-time-zone probe
)) hi-utc-diff
)
143 (defun calendar-time-zone-daylight-rules (abs-date utc-diff
)
144 "Return daylight transition rule for ABS-DATE, UTC-DIFF sec offset from UTC.
145 ABS-DATE must specify a day that contains a daylight savings transition.
146 The result has the proper form for calendar-daylight-savings-starts'."
147 (let* ((date (calendar-gregorian-from-absolute abs-date
))
148 (weekday (% abs-date
7))
149 (m (extract-calendar-month date
))
150 (d (extract-calendar-day date
))
151 (y (extract-calendar-year date
))
152 (last (calendar-last-day-of-month m y
))
156 (list (list 'list m d
'year
))
157 ;; The first WEEKDAY of month M.
159 (list (list 'calendar-nth-named-day
1 weekday m
'year
)))
160 ;; The last WEEKDAY of month M.
162 (list (list 'calendar-nth-named-day -
1 weekday m
'year
)))
163 ;; The first WEEKDAY after day J of month M, for D-6 < J <= D.
165 (calendar-for-loop j from
(max 2 (- d
6)) to
(min d
(- last
8)) do
168 (list 'calendar-nth-named-day
1 weekday m
'year j
)
171 ;; 01-01 and 07-01 for this year's Persian calendar.
172 (if (and (= m
3) (<= 20 d
) (<= d
21))
173 '((calendar-gregorian-from-absolute
174 (calendar-absolute-from-persian
175 (list 1 1 (- year
621))))))
176 (if (and (= m
9) (<= 22 d
) (<= d
23))
177 '((calendar-gregorian-from-absolute
178 (calendar-absolute-from-persian
179 (list 7 1 (- year
621))))))))
180 (prevday-sec (- -
1 utc-diff
)) ;; last sec of previous local day
182 ;; Scan through the next few years until only one rule remains.
184 (let ((rules candidate-rules
)
190 ;; The following is much faster than
191 ;; (calendar-absolute-from-gregorian (eval rule)).
192 (cond ((eq (car rule
) 'calendar-nth-named-day
)
193 (eval (cons 'calendar-nth-named-absday
(cdr rule
))))
194 ((eq (car rule
) 'calendar-gregorian-from-absolute
)
195 (eval (car (cdr rule
))))
196 (t (let ((g (eval rule
)))
197 (calendar-absolute-from-gregorian g
))))))
200 (calendar-time-from-absolute date prevday-sec
))
202 (calendar-time-from-absolute (1+ date
) prevday-sec
)))
203 (setq new-rules
(cons rule new-rules
)))
204 (setq rules
(cdr rules
))))
205 ;; If no rules remain, just use the first candidate rule;
206 ;; it's wrong in general, but it's right for at least one year.
207 (setq candidate-rules
(if new-rules
(nreverse new-rules
)
208 (list (car candidate-rules
))))
209 (setq year
(1+ year
))
210 (cdr candidate-rules
)))
211 (car candidate-rules
)))
213 ;; TODO it might be better to extract this information directly from
214 ;; the system timezone database. But cross-platform...?
216 ;; http://lists.gnu.org/archive/html/emacs-pretest-bug/2006-11/msg00060.html
217 (defun calendar-dst-find-data (&optional time
)
218 "Find data on the first Daylight Saving Time transitions after TIME.
219 TIME defaults to `current-time'. Return value is as described
220 for `calendar-current-time-zone'."
221 (let* ((t0 (or time
(current-time)))
222 (t0-zone (current-time-zone t0
))
223 (t0-utc-diff (car t0-zone
))
224 (t0-name (car (cdr t0-zone
))))
225 (if (not t0-utc-diff
)
226 ;; Little or no time zone information is available.
227 (list nil nil t0-name t0-name nil nil nil nil
)
228 (let* ((t1 (calendar-next-time-zone-transition t0
))
229 (t2 (and t1
(calendar-next-time-zone-transition t1
))))
231 ;; This locale does not have daylight savings time.
232 (list (/ t0-utc-diff
60) 0 t0-name t0-name nil nil
0 0)
233 ;; Use heuristics to find daylight savings parameters.
234 (let* ((t1-zone (current-time-zone t1
))
235 (t1-utc-diff (car t1-zone
))
236 (t1-name (car (cdr t1-zone
)))
237 (t1-date-sec (calendar-absolute-from-time t1 t0-utc-diff
))
238 (t2-date-sec (calendar-absolute-from-time t2 t1-utc-diff
))
239 ;; TODO When calendar-dst-check-each-year-flag is non-nil,
240 ;; the rules can be simpler than they currently are.
241 (t1-rules (calendar-time-zone-daylight-rules
242 (car t1-date-sec
) t0-utc-diff
))
243 (t2-rules (calendar-time-zone-daylight-rules
244 (car t2-date-sec
) t1-utc-diff
))
245 (t1-time (/ (cdr t1-date-sec
) 60))
246 (t2-time (/ (cdr t2-date-sec
) 60)))
248 (/ (min t0-utc-diff t1-utc-diff
) 60)
250 (/ (abs (- t0-utc-diff t1-utc-diff
)) 60)
251 (if (< t0-utc-diff t1-utc-diff
)
252 (list t0-name t1-name t1-rules t2-rules t1-time t2-time
)
253 (list t1-name t0-name t2-rules t1-rules t2-time t1-time
)
256 (defvar calendar-dst-transition-cache nil
257 "Internal cal-dst variable storing date of Daylight Saving Time transitions.
258 Value is a list with elements of the form (YEAR START END), where
259 START and END are expressions that when evaluated return the
260 start and end dates (respectively) for DST in YEAR. Used by the
261 function `calendar-dst-find-startend'.")
263 (defun calendar-dst-find-startend (year)
264 "Find the dates in YEAR on which Daylight Saving Time starts and ends.
265 Returns a list (YEAR START END), where START and END are
266 expressions that when evaluated return the start and end dates,
267 respectively. This function first attempts to use pre-calculated
268 data from `calendar-dst-transition-cache', otherwise it calls
269 `calendar-dst-find-data' (and adds the results to the cache)."
270 (let ((e (assoc year calendar-dst-transition-cache
))
274 (setq e
(calendar-dst-find-data (encode-time 1 0 0 1 1 year
))
276 e
(list year f
(nth 5 e
))
277 calendar-dst-transition-cache
278 (append calendar-dst-transition-cache
(list e
)))
281 (defun calendar-current-time-zone ()
282 "Return UTC difference, dst offset, names and rules for current time zone.
284 Returns (UTC-DIFF DST-OFFSET STD-ZONE DST-ZONE DST-STARTS DST-ENDS
285 DST-STARTS-TIME DST-ENDS-TIME), based on a heuristic probing of what the
288 UTC-DIFF is an integer specifying the number of minutes difference between
289 standard time in the current time zone and Coordinated Universal Time
290 (Greenwich Mean Time). A negative value means west of Greenwich.
291 DST-OFFSET is an integer giving the daylight savings time offset in minutes.
292 STD-ZONE is a string giving the name of the time zone when no seasonal time
293 adjustment is in effect.
294 DST-ZONE is a string giving the name of the time zone when there is a seasonal
295 time adjustment in effect.
296 DST-STARTS and DST-ENDS are sexps in the variable `year' giving the daylight
297 savings time start and end rules, in the form expected by
298 `calendar-daylight-savings-starts'.
299 DST-STARTS-TIME and DST-ENDS-TIME are integers giving the number of minutes
300 after midnight that daylight savings time starts and ends.
302 If the local area does not use a seasonal time adjustment, STD-ZONE and
303 DST-ZONE are equal, and all the DST-* integer variables are 0.
305 Some operating systems cannot provide all this information to Emacs; in this
306 case, `calendar-current-time-zone' returns a list containing nil for the data
308 (unless calendar-current-time-zone-cache
309 (setq calendar-current-time-zone-cache
(calendar-dst-find-data))))
311 ;;; The following eight defvars relating to daylight savings time should NOT be
312 ;;; marked to go into loaddefs.el where they would be evaluated when Emacs is
313 ;;; dumped. These variables' appropriate values depend on the conditions under
314 ;;; which the code is INVOKED; so it's inappropriate to initialize them when
315 ;;; Emacs is dumped---they should be initialized when calendar.el is loaded.
316 ;;; They default to US Eastern time if time zone info is not available.
318 (calendar-current-time-zone)
320 (defvar calendar-time-zone
(or (car calendar-current-time-zone-cache
) -
300)
321 "*Number of minutes difference between local standard time at
322 `calendar-location-name' and Coordinated Universal (Greenwich) Time. For
323 example, -300 for New York City, -480 for Los Angeles.")
325 (defvar calendar-daylight-time-offset
326 (or (car (cdr calendar-current-time-zone-cache
)) 60)
327 "*Number of minutes difference between daylight savings and standard time.
329 If the locale never uses daylight savings time, set this to 0.")
331 (defvar calendar-standard-time-zone-name
332 (or (car (nthcdr 2 calendar-current-time-zone-cache
)) "EST")
333 "*Abbreviated name of standard time zone at `calendar-location-name'.
334 For example, \"EST\" in New York City, \"PST\" for Los Angeles.")
336 (defvar calendar-daylight-time-zone-name
337 (or (car (nthcdr 3 calendar-current-time-zone-cache
)) "EDT")
338 "*Abbreviated name of daylight-savings time zone at `calendar-location-name'.
339 For example, \"EDT\" in New York City, \"PDT\" for Los Angeles.")
342 (defun calendar-dst-starts (year)
343 "Return the date of YEAR on which Daylight Saving Time starts.
344 This function respects the value of `calendar-dst-check-each-year-flag'."
345 (or (let ((expr (if calendar-dst-check-each-year-flag
346 (cadr (calendar-dst-find-startend year
))
347 (nth 4 calendar-current-time-zone-cache
))))
348 (if expr
(eval expr
)))
349 ;; New US rules commencing 2007. ftp://elsie.nci.nih.gov/pub/.
350 (and (not (zerop calendar-daylight-time-offset
))
351 (calendar-nth-named-day 2 0 3 year
))))
353 (defun calendar-dst-ends (year)
354 "Return the date of YEAR on which Daylight Saving Time ends.
355 This function respects the value of `calendar-dst-check-each-year-flag'."
356 (or (let ((expr (if calendar-dst-check-each-year-flag
357 (nth 2 (calendar-dst-find-startend year
))
358 (nth 5 calendar-current-time-zone-cache
))))
359 (if expr
(eval expr
)))
360 ;; New US rules commencing 2007. ftp://elsie.nci.nih.gov/pub/.
361 (and (not (zerop calendar-daylight-time-offset
))
362 (calendar-nth-named-day 1 0 11 year
))))
366 (put 'calendar-daylight-savings-starts
'risky-local-variable t
)
367 (defvar calendar-daylight-savings-starts
368 '(calendar-dst-starts year
)
369 "*Sexp giving the date on which daylight savings time starts.
370 This is an expression in the variable `year' whose value gives the Gregorian
371 date in the form (month day year) on which daylight savings time starts. It is
372 used to determine the starting date of daylight savings time for the holiday
373 list and for correcting times of day in the solar and lunar calculations.
375 For example, if daylight savings time is mandated to start on October 1,
376 you would set `calendar-daylight-savings-starts' to
380 If it starts on the first Sunday in April, you would set it to
382 '(calendar-nth-named-day 1 0 4 year)
384 If the locale never uses daylight savings time, set this to nil.")
387 (put 'calendar-daylight-savings-ends
'risky-local-variable t
)
388 (defvar calendar-daylight-savings-ends
389 '(calendar-dst-ends year
)
390 "*Sexp giving the date on which daylight savings time ends.
391 This is an expression in the variable `year' whose value gives the Gregorian
392 date in the form (month day year) on which daylight savings time ends. It is
393 used to determine the starting date of daylight savings time for the holiday
394 list and for correcting times of day in the solar and lunar calculations.
396 For example, if daylight savings time ends on the last Sunday in October:
398 '(calendar-nth-named-day -1 0 10 year)
400 If the locale never uses daylight savings time, set this to nil.")
402 (defvar calendar-daylight-savings-starts-time
403 (or (car (nthcdr 6 calendar-current-time-zone-cache
)) 120)
404 "*Number of minutes after midnight that daylight savings time starts.")
406 (defvar calendar-daylight-savings-ends-time
407 (or (car (nthcdr 7 calendar-current-time-zone-cache
))
408 calendar-daylight-savings-starts-time
)
409 "*Number of minutes after midnight that daylight savings time ends.")
411 (defun dst-in-effect (date)
412 "True if on absolute DATE daylight savings time is in effect.
413 Fractional part of DATE is local standard time of day."
414 (let* ((year (extract-calendar-year
415 (calendar-gregorian-from-absolute (floor date
))))
416 (dst-starts-gregorian (eval calendar-daylight-savings-starts
))
417 (dst-ends-gregorian (eval calendar-daylight-savings-ends
))
418 (dst-starts (and dst-starts-gregorian
419 (+ (calendar-absolute-from-gregorian
420 dst-starts-gregorian
)
421 (/ calendar-daylight-savings-starts-time
423 (dst-ends (and dst-ends-gregorian
424 (+ (calendar-absolute-from-gregorian
426 (/ (- calendar-daylight-savings-ends-time
427 calendar-daylight-time-offset
)
429 (and dst-starts dst-ends
430 (if (< dst-starts dst-ends
)
431 (and (<= dst-starts date
) (< date dst-ends
))
432 (or (<= dst-starts date
) (< date dst-ends
))))))
434 (defun dst-adjust-time (date time
&optional style
)
435 "Adjust, to account for dst on DATE, decimal fraction standard TIME.
436 Returns a list (date adj-time zone) where `date' and `adj-time' are the values
437 adjusted for `zone'; here `date' is a list (month day year), `adj-time' is a
438 decimal fraction time, and `zone' is a string.
440 Optional parameter STYLE forces the result time to be standard time when its
441 value is 'standard and daylight savings time (if available) when its value is
444 Conversion to daylight savings time is done according to
445 `calendar-daylight-savings-starts', `calendar-daylight-savings-ends',
446 `calendar-daylight-savings-starts-time',
447 `calendar-daylight-savings-ends-time', and
448 `calendar-daylight-savings-offset'."
450 (let* ((rounded-abs-date (+ (calendar-absolute-from-gregorian date
)
451 (/ (round (* 60 time
)) 60.0 24.0)))
452 (dst (dst-in-effect rounded-abs-date
))
454 calendar-daylight-time-zone-name
455 calendar-standard-time-zone-name
))
456 (time (+ rounded-abs-date
457 (if dst
(/ calendar-daylight-time-offset
24.0 60.0) 0))))
458 (list (calendar-gregorian-from-absolute (truncate time
))
459 (* 24.0 (- time
(truncate time
)))
464 ;;; arch-tag: a141d204-213c-4ca5-bdc6-f9df3aa92aad
465 ;;; cal-dst.el ends here