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1 ;;; cal-dst.el --- calendar functions for daylight saving rules
3 ;; Copyright (C) 1993-1996, 2001-2017 Free Software Foundation, Inc.
5 ;; Author: Paul Eggert <eggert@twinsun.com>
6 ;; Edward M. Reingold <reingold@cs.uiuc.edu>
7 ;; Maintainer: Glenn Morris <rgm@gnu.org>
8 ;; Keywords: calendar
9 ;; Human-Keywords: daylight saving time, calendar, diary, holidays
10 ;; Package: calendar
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 3 of the License, or
17 ;; (at your option) any later version.
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. If not, see <http://www.gnu.org/licenses/>.
27 ;;; Commentary:
29 ;; See calendar.el.
31 ;;; Code:
33 (require 'calendar)
36 (defgroup calendar-dst nil
37 "Options related to Daylight Saving Time."
38 :prefix "calendar-"
39 :group 'calendar)
42 (defcustom calendar-dst-check-each-year-flag t
43 "Non-nil means to check each year for DST transitions as needed.
44 Otherwise assume the next two transitions found after the
45 current date apply to all years. This is faster, but not always
46 correct, since the dates of daylight saving transitions sometimes
47 change."
48 :type 'boolean
49 :version "22.1"
50 :group 'calendar-dst)
52 ;;;###autoload
53 (put 'calendar-daylight-savings-starts 'risky-local-variable t)
54 (defcustom calendar-daylight-savings-starts '(calendar-dst-starts year)
55 "Sexp giving the date on which daylight saving time starts.
56 This is an expression in the variable `year' whose value gives the Gregorian
57 date in the form (month day year) on which daylight saving time starts. It is
58 used to determine the starting date of daylight saving time for the holiday
59 list and for correcting times of day in the solar and lunar calculations.
61 For example, if daylight saving time is mandated to start on October 1,
62 you would set `calendar-daylight-savings-starts' to
64 '(10 1 year)
66 If it starts on the first Sunday in April, you would set it to
68 '(calendar-nth-named-day 1 0 4 year)
70 If the locale never uses daylight saving time, set this to nil."
71 :type 'sexp
72 :group 'calendar-dst)
74 ;;;###autoload
75 (put 'calendar-daylight-savings-ends 'risky-local-variable t)
76 (defcustom calendar-daylight-savings-ends '(calendar-dst-ends year)
77 "Sexp giving the date on which daylight saving time ends.
78 This is an expression in the variable `year' whose value gives the Gregorian
79 date in the form (month day year) on which daylight saving time ends. It is
80 used to determine the starting date of daylight saving time for the holiday
81 list and for correcting times of day in the solar and lunar calculations.
83 For example, if daylight saving time ends on the last Sunday in October:
85 (calendar-nth-named-day -1 0 10 year)
87 If the locale never uses daylight saving time, set this to nil."
88 :type 'sexp
89 :group 'calendar-dst)
91 ;;; More defcustoms below.
94 (defvar calendar-current-time-zone-cache nil
95 "Cache for result of `calendar-current-time-zone'.")
96 ;; It gets eval'd, eg by calendar-dst-starts.
97 ;;;###autoload
98 (put 'calendar-current-time-zone-cache 'risky-local-variable t)
100 (defvar calendar-system-time-basis
101 (calendar-absolute-from-gregorian '(1 1 1970))
102 "Absolute date of starting date of system clock.")
104 (defun calendar-absolute-from-time (x utc-diff)
105 "Absolute local date of time X; local time is UTC-DIFF seconds from UTC.
107 X is (HIGH . LOW) or (HIGH LOW . IGNORED) where HIGH and LOW are the
108 high and low 16 bits, respectively, of the number of seconds since
109 1970-01-01 00:00:00 UTC, ignoring leap seconds.
111 Returns the pair (ABS-DATE . SECONDS) where SECONDS after local midnight on
112 absolute date ABS-DATE is the equivalent moment to X."
113 (let* ((h (car x))
114 (xtail (cdr x))
115 (l (+ utc-diff (if (numberp xtail) xtail (car xtail))))
116 (u (+ (* 512 (mod h 675)) (floor l 128))))
117 ;; Overflow is a terrible thing!
118 (cons (+ calendar-system-time-basis
119 ;; floor((2^16 h +l) / (60*60*24))
120 (* 512 (floor h 675)) (floor u 675))
121 ;; (2^16 h +l) mod (60*60*24)
122 (+ (* (mod u 675) 128) (mod l 128)))))
124 (defun calendar-time-from-absolute (abs-date s)
125 "Time of absolute date ABS-DATE, S seconds after midnight.
127 Returns the list (HIGH LOW) where HIGH and LOW are the high and low
128 16 bits, respectively, of the number of seconds 1970-01-01 00:00:00 UTC,
129 ignoring leap seconds, that is the equivalent moment to S seconds after
130 midnight UTC on absolute date ABS-DATE."
131 (let* ((a (- abs-date calendar-system-time-basis))
132 (u (+ (* 163 (mod a 512)) (floor s 128))))
133 ;; Overflow is a terrible thing!
134 (list
135 ;; floor((60*60*24*a + s) / 2^16)
136 (+ a (* 163 (floor a 512)) (floor u 512))
137 ;; (60*60*24*a + s) mod 2^16
138 (+ (* 128 (mod u 512)) (mod s 128)))))
140 (defun calendar-next-time-zone-transition (time)
141 "Return the time of the next time zone transition after TIME.
142 Both TIME and the result are acceptable arguments to `current-time-zone'.
143 Return nil if no such transition can be found."
144 (let* ((base 65536) ; 2^16 = base of current-time output
145 (quarter-multiple 120) ; approx = (seconds per quarter year) / base
146 (time-zone (current-time-zone time))
147 (time-utc-diff (car time-zone))
149 hi-zone
150 (hi-utc-diff time-utc-diff)
151 (quarters '(2 1 3)))
152 ;; Heuristic: probe the time zone offset in the next three calendar
153 ;; quarters, looking for a time zone offset different from TIME.
154 (while (and quarters (eq time-utc-diff hi-utc-diff))
155 (setq hi (cons (+ (car time) (* (car quarters) quarter-multiple)) 0)
156 hi-zone (current-time-zone hi)
157 hi-utc-diff (car hi-zone)
158 quarters (cdr quarters)))
159 (and
160 time-utc-diff
161 hi-utc-diff
162 (not (eq time-utc-diff hi-utc-diff))
163 ;; Now HI is after the next time zone transition.
164 ;; Set LO to TIME, and then binary search to increase LO and decrease HI
165 ;; until LO is just before and HI is just after the time zone transition.
166 (let* ((tail (cdr time))
167 (lo (cons (car time) (if (numberp tail) tail (car tail))))
168 probe)
169 (while
170 ;; Set PROBE to halfway between LO and HI, rounding down.
171 ;; If PROBE equals LO, we are done.
172 (let* ((lsum (+ (cdr lo) (cdr hi)))
173 (hsum (+ (car lo) (car hi) (/ lsum base)))
174 (hsumodd (logand 1 hsum)))
175 (setq probe (cons (/ (- hsum hsumodd) 2)
176 (/ (+ (* hsumodd base) (% lsum base)) 2)))
177 (not (equal lo probe)))
178 ;; Set either LO or HI to PROBE, depending on probe results.
179 (if (eq (car (current-time-zone probe)) hi-utc-diff)
180 (setq hi probe)
181 (setq lo probe)))
182 (setcdr hi (list (cdr hi)))
183 hi))))
185 (autoload 'calendar-persian-to-absolute "cal-persia")
187 (defun calendar-time-zone-daylight-rules (abs-date utc-diff)
188 "Return daylight transition rule for ABS-DATE, UTC-DIFF sec offset from UTC.
189 ABS-DATE must specify a day that contains a daylight saving transition.
190 The result has the proper form for `calendar-daylight-savings-starts'."
191 (let* ((date (calendar-gregorian-from-absolute abs-date))
192 (weekday (% abs-date 7))
193 (m (calendar-extract-month date))
194 (d (calendar-extract-day date))
195 (y (calendar-extract-year date))
196 (last (calendar-last-day-of-month m y))
197 j rlist
198 (candidate-rules ; these return Gregorian dates
199 (append
200 ;; Day D of month M.
201 `((list ,m ,d year))
202 ;; The first WEEKDAY of month M.
203 (if (< d 8)
204 `((calendar-nth-named-day 1 ,weekday ,m year)))
205 ;; The last WEEKDAY of month M.
206 (if (> d (- last 7))
207 `((calendar-nth-named-day -1 ,weekday ,m year)))
208 (progn
209 ;; The first WEEKDAY after day J of month M, for D-6 < J <= D.
210 (setq j (1- (max 2 (- d 6))))
211 (while (<= (setq j (1+ j)) (min d (- last 8)))
212 (push `(calendar-nth-named-day 1 ,weekday ,m year ,j) rlist))
213 rlist)
214 ;; 01-01 and 07-01 for this year's Persian calendar.
215 ;; FIXME what does the Persian calendar have to do with this?
216 (and (= m 3) (memq d '(20 21))
217 '((calendar-gregorian-from-absolute
218 (calendar-persian-to-absolute `(1 1 ,(- year 621))))))
219 (and (= m 9) (memq d '(22 23))
220 '((calendar-gregorian-from-absolute
221 (calendar-persian-to-absolute `(7 1 ,(- year 621))))))))
222 (prevday-sec (- -1 utc-diff)) ; last sec of previous local day
223 (year (1+ y))
224 new-rules)
225 ;; Scan through the next few years until only one rule remains.
226 (while (cdr candidate-rules)
227 (dolist (rule candidate-rules)
228 ;; The rule we return should give a Gregorian date, but here
229 ;; we require an absolute date. The following is for efficiency.
230 (setq date (cond ((eq (car rule) 'calendar-nth-named-day)
231 (eval (cons 'calendar-nth-named-absday (cdr rule))))
232 ((eq (car rule) 'calendar-gregorian-from-absolute)
233 (eval (cadr rule)))
234 (t (calendar-absolute-from-gregorian (eval rule)))))
235 (or (equal (current-time-zone
236 (calendar-time-from-absolute date prevday-sec))
237 (current-time-zone
238 (calendar-time-from-absolute (1+ date) prevday-sec)))
239 (setq new-rules (cons rule new-rules))))
240 ;; If no rules remain, just use the first candidate rule;
241 ;; it's wrong in general, but it's right for at least one year.
242 (setq candidate-rules (if new-rules (nreverse new-rules)
243 (list (car candidate-rules)))
244 new-rules nil
245 year (1+ year)))
246 (car candidate-rules)))
248 ;; TODO it might be better to extract this information directly from
249 ;; the system timezone database. But cross-platform...?
250 ;; See thread
251 ;; http://lists.gnu.org/archive/html/emacs-pretest-bug/2006-11/msg00060.html
252 (defun calendar-dst-find-data (&optional time)
253 "Find data on the first daylight saving time transitions after TIME.
254 TIME defaults to `current-time'. Return value is as described
255 for `calendar-current-time-zone'."
256 (let* ((t0 (or time (current-time)))
257 (t0-zone (current-time-zone t0))
258 (t0-utc-diff (car t0-zone))
259 (t0-name (cadr t0-zone)))
260 (if (not t0-utc-diff)
261 ;; Little or no time zone information is available.
262 (list nil nil t0-name t0-name nil nil nil nil)
263 (let* ((t1 (calendar-next-time-zone-transition t0))
264 (t2 (and t1 (calendar-next-time-zone-transition t1))))
265 (if (not t2)
266 ;; This locale does not have daylight saving time.
267 (list (/ t0-utc-diff 60) 0 t0-name t0-name nil nil 0 0)
268 ;; Use heuristics to find daylight saving parameters.
269 (let* ((t1-zone (current-time-zone t1))
270 (t1-utc-diff (car t1-zone))
271 (t1-name (cadr t1-zone))
272 (t1-date-sec (calendar-absolute-from-time t1 t0-utc-diff))
273 (t2-date-sec (calendar-absolute-from-time t2 t1-utc-diff))
274 ;; TODO When calendar-dst-check-each-year-flag is non-nil,
275 ;; the rules can be simpler than they currently are.
276 (t1-rules (calendar-time-zone-daylight-rules
277 (car t1-date-sec) t0-utc-diff))
278 (t2-rules (calendar-time-zone-daylight-rules
279 (car t2-date-sec) t1-utc-diff))
280 (t1-time (/ (cdr t1-date-sec) 60))
281 (t2-time (/ (cdr t2-date-sec) 60)))
282 (cons
283 (/ (min t0-utc-diff t1-utc-diff) 60)
284 (cons
285 (/ (abs (- t0-utc-diff t1-utc-diff)) 60)
286 (if (< t0-utc-diff t1-utc-diff)
287 (list t0-name t1-name t1-rules t2-rules t1-time t2-time)
288 (list t1-name t0-name t2-rules t1-rules t2-time t1-time)
289 )))))))))
291 (defvar calendar-dst-transition-cache nil
292 "Internal cal-dst variable storing date of daylight saving time transitions.
293 Value is a list with elements of the form (YEAR START END), where
294 START and END are expressions that when evaluated return the
295 start and end dates (respectively) for DST in YEAR. Used by the
296 function `calendar-dst-find-startend'.")
298 (defun calendar-dst-find-startend (year)
299 "Find the dates in YEAR on which daylight saving time starts and ends.
300 Returns a list (YEAR START END), where START and END are
301 expressions that when evaluated return the start and end dates,
302 respectively. This function first attempts to use pre-calculated
303 data from `calendar-dst-transition-cache', otherwise it calls
304 `calendar-dst-find-data' (and adds the results to the cache).
305 If dates in YEAR cannot be handled by `encode-time' (e.g. if they
306 are too large to be represented as a lisp integer), then rather
307 than an error this function returns the result appropriate for
308 the current year."
309 (let ((e (assoc year calendar-dst-transition-cache))
311 (or e
312 (progn
313 (setq e (calendar-dst-find-data
314 (condition-case nil
315 (encode-time 1 0 0 1 1 year)
316 (error
317 (encode-time 1 0 0 1 1 (nth 5 (decode-time))))))
318 f (nth 4 e)
319 e (list year f (nth 5 e))
320 calendar-dst-transition-cache
321 (append calendar-dst-transition-cache (list e)))
322 e))))
324 (defun calendar-current-time-zone ()
325 "Return UTC difference, dst offset, names and rules for current time zone.
327 Returns (UTC-DIFF DST-OFFSET STD-ZONE DST-ZONE DST-STARTS DST-ENDS
328 DST-STARTS-TIME DST-ENDS-TIME), based on a heuristic probing of what the
329 system knows:
331 UTC-DIFF is an integer specifying the number of minutes difference between
332 standard time in the current time zone and Coordinated Universal Time
333 (Greenwich Mean Time). A negative value means west of Greenwich.
334 DST-OFFSET is an integer giving the daylight saving time offset in minutes.
335 STD-ZONE is a string giving the name of the time zone when no seasonal time
336 adjustment is in effect.
337 DST-ZONE is a string giving the name of the time zone when there is a seasonal
338 time adjustment in effect.
339 DST-STARTS and DST-ENDS are sexps in the variable `year' giving the daylight
340 saving time start and end rules, in the form expected by
341 `calendar-daylight-savings-starts'.
342 DST-STARTS-TIME and DST-ENDS-TIME are integers giving the number of minutes
343 after midnight that daylight saving time starts and ends.
345 If the local area does not use a seasonal time adjustment, STD-ZONE and
346 DST-ZONE are equal, and all the DST-* integer variables are 0.
348 Some operating systems cannot provide all this information to Emacs; in this
349 case, `calendar-current-time-zone' returns a list containing nil for the data
350 it can't find."
351 (or calendar-current-time-zone-cache
352 (setq calendar-current-time-zone-cache (calendar-dst-find-data))))
355 ;; Following options should be set based on conditions when the code
356 ;; is invoked, so are not suitable for dumping into loaddefs.el. They
357 ;; default to US Eastern time if time zone info is not available.
359 (calendar-current-time-zone)
361 (defcustom calendar-time-zone (or (car calendar-current-time-zone-cache) -300)
362 "Number of minutes difference between local standard time and UTC.
363 For example, -300 for New York City, -480 for Los Angeles."
364 :type 'integer
365 :group 'calendar-dst)
367 (defcustom calendar-daylight-time-offset
368 (or (cadr calendar-current-time-zone-cache) 60)
369 "Number of minutes difference between daylight saving and standard time.
370 If the locale never uses daylight saving time, set this to 0."
371 :type 'integer
372 :group 'calendar-dst)
374 (defcustom calendar-standard-time-zone-name
375 (or (nth 2 calendar-current-time-zone-cache) "EST")
376 "Abbreviated name of standard time zone at `calendar-location-name'.
377 For example, \"EST\" in New York City, \"PST\" for Los Angeles."
378 :type 'string
379 :group 'calendar-dst)
381 (defcustom calendar-daylight-time-zone-name
382 (or (nth 3 calendar-current-time-zone-cache) "EDT")
383 "Abbreviated name of daylight saving time zone at `calendar-location-name'.
384 For example, \"EDT\" in New York City, \"PDT\" for Los Angeles."
385 :type 'string
386 :group 'calendar-dst)
388 (defcustom calendar-daylight-savings-starts-time
389 (or (nth 6 calendar-current-time-zone-cache) 120)
390 "Number of minutes after midnight that daylight saving time starts."
391 :type 'integer
392 :group 'calendar-dst)
394 (defcustom calendar-daylight-savings-ends-time
395 (or (nth 7 calendar-current-time-zone-cache)
396 calendar-daylight-savings-starts-time)
397 "Number of minutes after midnight that daylight saving time ends."
398 :type 'integer
399 :group 'calendar-dst)
402 (defun calendar-dst-starts (year)
403 "Return the date of YEAR on which daylight saving time starts.
404 This function respects the value of `calendar-dst-check-each-year-flag'."
405 (or (let ((expr (if calendar-dst-check-each-year-flag
406 (cadr (calendar-dst-find-startend year))
407 (nth 4 calendar-current-time-zone-cache))))
408 (if expr (eval expr)))
409 ;; New US rules commencing 2007. ftp://elsie.nci.nih.gov/pub/.
410 (and (not (zerop calendar-daylight-time-offset))
411 (calendar-nth-named-day 2 0 3 year))))
413 (defun calendar-dst-ends (year)
414 "Return the date of YEAR on which daylight saving time ends.
415 This function respects the value of `calendar-dst-check-each-year-flag'."
416 (or (let ((expr (if calendar-dst-check-each-year-flag
417 (nth 2 (calendar-dst-find-startend year))
418 (nth 5 calendar-current-time-zone-cache))))
419 (if expr (eval expr)))
420 ;; New US rules commencing 2007. ftp://elsie.nci.nih.gov/pub/.
421 (and (not (zerop calendar-daylight-time-offset))
422 (calendar-nth-named-day 1 0 11 year))))
424 ;; used by calc, solar.
425 (defun dst-in-effect (date)
426 "True if on absolute DATE daylight saving time is in effect.
427 Fractional part of DATE is local standard time of day."
428 (let* ((year (calendar-extract-year
429 (calendar-gregorian-from-absolute (floor date))))
430 (dst-starts-gregorian (eval calendar-daylight-savings-starts))
431 (dst-ends-gregorian (eval calendar-daylight-savings-ends))
432 (dst-starts (and dst-starts-gregorian
433 (+ (calendar-absolute-from-gregorian
434 dst-starts-gregorian)
435 (/ calendar-daylight-savings-starts-time
436 60.0 24.0))))
437 (dst-ends (and dst-ends-gregorian
438 (+ (calendar-absolute-from-gregorian
439 dst-ends-gregorian)
440 (/ (- calendar-daylight-savings-ends-time
441 calendar-daylight-time-offset)
442 60.0 24.0)))))
443 (and dst-starts dst-ends
444 (if (< dst-starts dst-ends)
445 (and (<= dst-starts date) (< date dst-ends))
446 (or (<= dst-starts date) (< date dst-ends))))))
448 ;; used by calc, lunar, solar.
449 (defun dst-adjust-time (date time)
450 "Adjust, to account for dst on DATE, decimal fraction standard TIME.
451 Returns a list (date adj-time zone) where `date' and `adj-time' are the values
452 adjusted for `zone'; here `date' is a list (month day year), `adj-time' is a
453 decimal fraction time, and `zone' is a string.
455 Conversion to daylight saving time is done according to
456 `calendar-daylight-savings-starts', `calendar-daylight-savings-ends',
457 `calendar-daylight-savings-starts-time',
458 `calendar-daylight-savings-ends-time', and `calendar-daylight-time-offset'."
459 (let* ((rounded-abs-date (+ (calendar-absolute-from-gregorian date)
460 (/ (round (* 60 time)) 60.0 24.0)))
461 (dst (dst-in-effect rounded-abs-date))
462 (time-zone (if dst
463 calendar-daylight-time-zone-name
464 calendar-standard-time-zone-name))
465 (time (+ rounded-abs-date
466 (if dst (/ calendar-daylight-time-offset 24.0 60.0) 0))))
467 (list (calendar-gregorian-from-absolute (truncate time))
468 (* 24.0 (- time (truncate time)))
469 time-zone)))
471 (provide 'cal-dst)
473 ;;; cal-dst.el ends here