1 ;;; cal-dst.el --- calendar functions for daylight saving 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 saving 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 saving time.
39 (defcustom calendar-dst-check-each-year-flag t
40 "Non-nil means to check each year for DST transitions as needed.
41 Otherwise assume the next two transitions found after the
42 current date apply to all years. This is faster, but not always
43 correct, since the dates of daylight saving transitions sometimes
49 (defvar calendar-current-time-zone-cache nil
50 "Cache for result of `calendar-current-time-zone'.")
51 (put 'calendar-current-time-zone-cache
'risky-local-variable t
)
53 (defvar calendar-system-time-basis
54 (calendar-absolute-from-gregorian '(1 1 1970))
55 "Absolute date of starting date of system clock.")
57 (defun calendar-absolute-from-time (x utc-diff
)
58 "Absolute local date of time X; local time is UTC-DIFF seconds from UTC.
60 X is (HIGH . LOW) or (HIGH LOW . IGNORED) where HIGH and LOW are the
61 high and low 16 bits, respectively, of the number of seconds since
62 1970-01-01 00:00:00 UTC, ignoring leap seconds.
64 Returns the pair (ABS-DATE . SECONDS) where SECONDS after local midnight on
65 absolute date ABS-DATE is the equivalent moment to X."
68 (l (+ utc-diff
(if (numberp xtail
) xtail
(car xtail
))))
69 (u (+ (* 512 (mod h
675)) (floor l
128))))
70 ;; Overflow is a terrible thing!
71 (cons (+ calendar-system-time-basis
72 ;; floor((2^16 h +l) / (60*60*24))
73 (* 512 (floor h
675)) (floor u
675))
74 ;; (2^16 h +l) mod (60*60*24)
75 (+ (* (mod u
675) 128) (mod l
128)))))
77 (defun calendar-time-from-absolute (abs-date s
)
78 "Time of absolute date ABS-DATE, S seconds after midnight.
80 Returns the list (HIGH LOW) where HIGH and LOW are the high and low
81 16 bits, respectively, of the number of seconds 1970-01-01 00:00:00 UTC,
82 ignoring leap seconds, that is the equivalent moment to S seconds after
83 midnight UTC on absolute date ABS-DATE."
84 (let* ((a (- abs-date calendar-system-time-basis
))
85 (u (+ (* 163 (mod a
512)) (floor s
128))))
86 ;; Overflow is a terrible thing!
88 ;; floor((60*60*24*a + s) / 2^16)
89 (+ a
(* 163 (floor a
512)) (floor u
512))
90 ;; (60*60*24*a + s) mod 2^16
91 (+ (* 128 (mod u
512)) (mod s
128)))))
93 (defun calendar-next-time-zone-transition (time)
94 "Return the time of the next time zone transition after TIME.
95 Both TIME and the result are acceptable arguments to current-time-zone.
96 Return nil if no such transition can be found."
97 (let* ((base 65536);; 2^16 = base of current-time output
98 (quarter-multiple 120);; approx = (seconds per quarter year) / base
99 (time-zone (current-time-zone time
))
100 (time-utc-diff (car time-zone
))
103 (hi-utc-diff time-utc-diff
)
105 ;; Heuristic: probe the time zone offset in the next three calendar
106 ;; quarters, looking for a time zone offset different from TIME.
107 (while (and quarters
(eq time-utc-diff hi-utc-diff
))
108 (setq hi
(cons (+ (car time
) (* (car quarters
) quarter-multiple
)) 0))
109 (setq hi-zone
(current-time-zone hi
))
110 (setq hi-utc-diff
(car hi-zone
))
111 (setq quarters
(cdr quarters
)))
115 (not (eq time-utc-diff hi-utc-diff
))
116 ;; Now HI is after the next time zone transition.
117 ;; Set LO to TIME, and then binary search to increase LO and decrease HI
118 ;; until LO is just before and HI is just after the time zone transition.
119 (let* ((tail (cdr time
))
120 (lo (cons (car time
) (if (numberp tail
) tail
(car tail
))))
123 ;; Set PROBE to halfway between LO and HI, rounding down.
124 ;; If PROBE equals LO, we are done.
125 (let* ((lsum (+ (cdr lo
) (cdr hi
)))
126 (hsum (+ (car lo
) (car hi
) (/ lsum base
)))
127 (hsumodd (logand 1 hsum
)))
128 (setq probe
(cons (/ (- hsum hsumodd
) 2)
129 (/ (+ (* hsumodd base
) (% lsum base
)) 2)))
130 (not (equal lo probe
)))
131 ;; Set either LO or HI to PROBE, depending on probe results.
132 (if (eq (car (current-time-zone probe
)) hi-utc-diff
)
137 (defun calendar-time-zone-daylight-rules (abs-date utc-diff
)
138 "Return daylight transition rule for ABS-DATE, UTC-DIFF sec offset from UTC.
139 ABS-DATE must specify a day that contains a daylight saving transition.
140 The result has the proper form for `calendar-daylight-savings-starts'."
141 (let* ((date (calendar-gregorian-from-absolute abs-date
))
142 (weekday (% abs-date
7))
143 (m (extract-calendar-month date
))
144 (d (extract-calendar-day date
))
145 (y (extract-calendar-year date
))
146 (last (calendar-last-day-of-month m y
))
150 (list (list 'list m d
'year
))
151 ;; The first WEEKDAY of month M.
153 (list (list 'calendar-nth-named-day
1 weekday m
'year
)))
154 ;; The last WEEKDAY of month M.
156 (list (list 'calendar-nth-named-day -
1 weekday m
'year
)))
157 ;; The first WEEKDAY after day J of month M, for D-6 < J <= D.
159 (calendar-for-loop j from
(max 2 (- d
6)) to
(min d
(- last
8)) do
162 (list 'calendar-nth-named-day
1 weekday m
'year j
)
165 ;; 01-01 and 07-01 for this year's Persian calendar.
166 (if (and (= m
3) (<= 20 d
) (<= d
21))
167 '((calendar-gregorian-from-absolute
168 (calendar-absolute-from-persian
169 (list 1 1 (- year
621))))))
170 (if (and (= m
9) (<= 22 d
) (<= d
23))
171 '((calendar-gregorian-from-absolute
172 (calendar-absolute-from-persian
173 (list 7 1 (- year
621))))))))
174 (prevday-sec (- -
1 utc-diff
)) ;; last sec of previous local day
176 ;; Scan through the next few years until only one rule remains.
178 (let ((rules candidate-rules
)
184 ;; The following is much faster than
185 ;; (calendar-absolute-from-gregorian (eval rule)).
186 (cond ((eq (car rule
) 'calendar-nth-named-day
)
187 (eval (cons 'calendar-nth-named-absday
(cdr rule
))))
188 ((eq (car rule
) 'calendar-gregorian-from-absolute
)
189 (eval (car (cdr rule
))))
190 (t (let ((g (eval rule
)))
191 (calendar-absolute-from-gregorian g
))))))
194 (calendar-time-from-absolute date prevday-sec
))
196 (calendar-time-from-absolute (1+ date
) prevday-sec
)))
197 (setq new-rules
(cons rule new-rules
)))
198 (setq rules
(cdr rules
))))
199 ;; If no rules remain, just use the first candidate rule;
200 ;; it's wrong in general, but it's right for at least one year.
201 (setq candidate-rules
(if new-rules
(nreverse new-rules
)
202 (list (car candidate-rules
))))
203 (setq year
(1+ year
))
204 (cdr candidate-rules
)))
205 (car candidate-rules
)))
207 ;; TODO it might be better to extract this information directly from
208 ;; the system timezone database. But cross-platform...?
210 ;; http://lists.gnu.org/archive/html/emacs-pretest-bug/2006-11/msg00060.html
211 (defun calendar-dst-find-data (&optional time
)
212 "Find data on the first daylight saving time transitions after TIME.
213 TIME defaults to `current-time'. Return value is as described
214 for `calendar-current-time-zone'."
215 (let* ((t0 (or time
(current-time)))
216 (t0-zone (current-time-zone t0
))
217 (t0-utc-diff (car t0-zone
))
218 (t0-name (car (cdr t0-zone
))))
219 (if (not t0-utc-diff
)
220 ;; Little or no time zone information is available.
221 (list nil nil t0-name t0-name nil nil nil nil
)
222 (let* ((t1 (calendar-next-time-zone-transition t0
))
223 (t2 (and t1
(calendar-next-time-zone-transition t1
))))
225 ;; This locale does not have daylight saving time.
226 (list (/ t0-utc-diff
60) 0 t0-name t0-name nil nil
0 0)
227 ;; Use heuristics to find daylight saving parameters.
228 (let* ((t1-zone (current-time-zone t1
))
229 (t1-utc-diff (car t1-zone
))
230 (t1-name (car (cdr t1-zone
)))
231 (t1-date-sec (calendar-absolute-from-time t1 t0-utc-diff
))
232 (t2-date-sec (calendar-absolute-from-time t2 t1-utc-diff
))
233 ;; TODO When calendar-dst-check-each-year-flag is non-nil,
234 ;; the rules can be simpler than they currently are.
235 (t1-rules (calendar-time-zone-daylight-rules
236 (car t1-date-sec
) t0-utc-diff
))
237 (t2-rules (calendar-time-zone-daylight-rules
238 (car t2-date-sec
) t1-utc-diff
))
239 (t1-time (/ (cdr t1-date-sec
) 60))
240 (t2-time (/ (cdr t2-date-sec
) 60)))
242 (/ (min t0-utc-diff t1-utc-diff
) 60)
244 (/ (abs (- t0-utc-diff t1-utc-diff
)) 60)
245 (if (< t0-utc-diff t1-utc-diff
)
246 (list t0-name t1-name t1-rules t2-rules t1-time t2-time
)
247 (list t1-name t0-name t2-rules t1-rules t2-time t1-time
)
250 (defvar calendar-dst-transition-cache nil
251 "Internal cal-dst variable storing date of daylight saving time transitions.
252 Value is a list with elements of the form (YEAR START END), where
253 START and END are expressions that when evaluated return the
254 start and end dates (respectively) for DST in YEAR. Used by the
255 function `calendar-dst-find-startend'.")
257 (defun calendar-dst-find-startend (year)
258 "Find the dates in YEAR on which daylight saving time starts and ends.
259 Returns a list (YEAR START END), where START and END are
260 expressions that when evaluated return the start and end dates,
261 respectively. This function first attempts to use pre-calculated
262 data from `calendar-dst-transition-cache', otherwise it calls
263 `calendar-dst-find-data' (and adds the results to the cache).
264 If dates in YEAR cannot be handled by `encode-time' (e.g. if they
265 are too large to be represented as a lisp integer), then rather
266 than an error this function returns the result appropriate for
268 (let ((e (assoc year calendar-dst-transition-cache
))
272 (setq e
(calendar-dst-find-data
274 (encode-time 1 0 0 1 1 year
)
276 (encode-time 1 0 0 1 1 (nth 5 (decode-time))))))
278 e
(list year f
(nth 5 e
))
279 calendar-dst-transition-cache
280 (append calendar-dst-transition-cache
(list e
)))
283 (defun calendar-current-time-zone ()
284 "Return UTC difference, dst offset, names and rules for current time zone.
286 Returns (UTC-DIFF DST-OFFSET STD-ZONE DST-ZONE DST-STARTS DST-ENDS
287 DST-STARTS-TIME DST-ENDS-TIME), based on a heuristic probing of what the
290 UTC-DIFF is an integer specifying the number of minutes difference between
291 standard time in the current time zone and Coordinated Universal Time
292 (Greenwich Mean Time). A negative value means west of Greenwich.
293 DST-OFFSET is an integer giving the daylight saving time offset in minutes.
294 STD-ZONE is a string giving the name of the time zone when no seasonal time
295 adjustment is in effect.
296 DST-ZONE is a string giving the name of the time zone when there is a seasonal
297 time adjustment in effect.
298 DST-STARTS and DST-ENDS are sexps in the variable `year' giving the daylight
299 saving time start and end rules, in the form expected by
300 `calendar-daylight-savings-starts'.
301 DST-STARTS-TIME and DST-ENDS-TIME are integers giving the number of minutes
302 after midnight that daylight saving time starts and ends.
304 If the local area does not use a seasonal time adjustment, STD-ZONE and
305 DST-ZONE are equal, and all the DST-* integer variables are 0.
307 Some operating systems cannot provide all this information to Emacs; in this
308 case, `calendar-current-time-zone' returns a list containing nil for the data
310 (unless calendar-current-time-zone-cache
311 (setq calendar-current-time-zone-cache
(calendar-dst-find-data))))
313 ;;; The following eight defvars relating to daylight saving time should NOT be
314 ;;; marked to go into loaddefs.el where they would be evaluated when Emacs is
315 ;;; dumped. These variables' appropriate values depend on the conditions under
316 ;;; which the code is INVOKED; so it's inappropriate to initialize them when
317 ;;; Emacs is dumped---they should be initialized when calendar.el is loaded.
318 ;;; They default to US Eastern time if time zone info is not available.
320 (calendar-current-time-zone)
322 (defvar calendar-time-zone
(or (car calendar-current-time-zone-cache
) -
300)
323 "*Number of minutes difference between local standard time at
324 `calendar-location-name' and Coordinated Universal (Greenwich) Time. For
325 example, -300 for New York City, -480 for Los Angeles.")
327 (defvar calendar-daylight-time-offset
328 (or (car (cdr calendar-current-time-zone-cache
)) 60)
329 "*Number of minutes difference between daylight saving and standard time.
331 If the locale never uses daylight saving time, set this to 0.")
333 (defvar calendar-standard-time-zone-name
334 (or (car (nthcdr 2 calendar-current-time-zone-cache
)) "EST")
335 "*Abbreviated name of standard time zone at `calendar-location-name'.
336 For example, \"EST\" in New York City, \"PST\" for Los Angeles.")
338 (defvar calendar-daylight-time-zone-name
339 (or (car (nthcdr 3 calendar-current-time-zone-cache
)) "EDT")
340 "*Abbreviated name of daylight saving time zone at `calendar-location-name'.
341 For example, \"EDT\" in New York City, \"PDT\" for Los Angeles.")
344 (defun calendar-dst-starts (year)
345 "Return the date of YEAR on which daylight saving time starts.
346 This function respects the value of `calendar-dst-check-each-year-flag'."
347 (or (let ((expr (if calendar-dst-check-each-year-flag
348 (cadr (calendar-dst-find-startend year
))
349 (nth 4 calendar-current-time-zone-cache
))))
350 (if expr
(eval expr
)))
351 ;; New US rules commencing 2007. ftp://elsie.nci.nih.gov/pub/.
352 (and (not (zerop calendar-daylight-time-offset
))
353 (calendar-nth-named-day 2 0 3 year
))))
355 (defun calendar-dst-ends (year)
356 "Return the date of YEAR on which daylight saving time ends.
357 This function respects the value of `calendar-dst-check-each-year-flag'."
358 (or (let ((expr (if calendar-dst-check-each-year-flag
359 (nth 2 (calendar-dst-find-startend year
))
360 (nth 5 calendar-current-time-zone-cache
))))
361 (if expr
(eval expr
)))
362 ;; New US rules commencing 2007. ftp://elsie.nci.nih.gov/pub/.
363 (and (not (zerop calendar-daylight-time-offset
))
364 (calendar-nth-named-day 1 0 11 year
))))
368 (put 'calendar-daylight-savings-starts
'risky-local-variable t
)
369 (defvar calendar-daylight-savings-starts
370 '(calendar-dst-starts year
)
371 "*Sexp giving the date on which daylight saving time starts.
372 This is an expression in the variable `year' whose value gives the Gregorian
373 date in the form (month day year) on which daylight saving time starts. It is
374 used to determine the starting date of daylight saving time for the holiday
375 list and for correcting times of day in the solar and lunar calculations.
377 For example, if daylight saving time is mandated to start on October 1,
378 you would set `calendar-daylight-savings-starts' to
382 If it starts on the first Sunday in April, you would set it to
384 '(calendar-nth-named-day 1 0 4 year)
386 If the locale never uses daylight saving time, set this to nil.")
389 (put 'calendar-daylight-savings-ends
'risky-local-variable t
)
390 (defvar calendar-daylight-savings-ends
391 '(calendar-dst-ends year
)
392 "*Sexp giving the date on which daylight saving time ends.
393 This is an expression in the variable `year' whose value gives the Gregorian
394 date in the form (month day year) on which daylight saving time ends. It is
395 used to determine the starting date of daylight saving time for the holiday
396 list and for correcting times of day in the solar and lunar calculations.
398 For example, if daylight saving time ends on the last Sunday in October:
400 '(calendar-nth-named-day -1 0 10 year)
402 If the locale never uses daylight saving time, set this to nil.")
404 (defvar calendar-daylight-savings-starts-time
405 (or (car (nthcdr 6 calendar-current-time-zone-cache
)) 120)
406 "*Number of minutes after midnight that daylight saving time starts.")
408 (defvar calendar-daylight-savings-ends-time
409 (or (car (nthcdr 7 calendar-current-time-zone-cache
))
410 calendar-daylight-savings-starts-time
)
411 "*Number of minutes after midnight that daylight saving time ends.")
413 (defun dst-in-effect (date)
414 "True if on absolute DATE daylight saving time is in effect.
415 Fractional part of DATE is local standard time of day."
416 (let* ((year (extract-calendar-year
417 (calendar-gregorian-from-absolute (floor date
))))
418 (dst-starts-gregorian (eval calendar-daylight-savings-starts
))
419 (dst-ends-gregorian (eval calendar-daylight-savings-ends
))
420 (dst-starts (and dst-starts-gregorian
421 (+ (calendar-absolute-from-gregorian
422 dst-starts-gregorian
)
423 (/ calendar-daylight-savings-starts-time
425 (dst-ends (and dst-ends-gregorian
426 (+ (calendar-absolute-from-gregorian
428 (/ (- calendar-daylight-savings-ends-time
429 calendar-daylight-time-offset
)
431 (and dst-starts dst-ends
432 (if (< dst-starts dst-ends
)
433 (and (<= dst-starts date
) (< date dst-ends
))
434 (or (<= dst-starts date
) (< date dst-ends
))))))
436 (defun dst-adjust-time (date time
&optional style
)
437 "Adjust, to account for dst on DATE, decimal fraction standard TIME.
438 Returns a list (date adj-time zone) where `date' and `adj-time' are the values
439 adjusted for `zone'; here `date' is a list (month day year), `adj-time' is a
440 decimal fraction time, and `zone' is a string.
442 Optional parameter STYLE forces the result time to be standard time when its
443 value is 'standard and daylight saving time (if available) when its value is
446 Conversion to daylight saving time is done according to
447 `calendar-daylight-savings-starts', `calendar-daylight-savings-ends',
448 `calendar-daylight-savings-starts-time',
449 `calendar-daylight-savings-ends-time', and
450 `calendar-daylight-savings-offset'."
452 (let* ((rounded-abs-date (+ (calendar-absolute-from-gregorian date
)
453 (/ (round (* 60 time
)) 60.0 24.0)))
454 (dst (dst-in-effect rounded-abs-date
))
456 calendar-daylight-time-zone-name
457 calendar-standard-time-zone-name
))
458 (time (+ rounded-abs-date
459 (if dst
(/ calendar-daylight-time-offset
24.0 60.0) 0))))
460 (list (calendar-gregorian-from-absolute (truncate time
))
461 (* 24.0 (- time
(truncate time
)))
466 ;;; arch-tag: a141d204-213c-4ca5-bdc6-f9df3aa92aad
467 ;;; cal-dst.el ends here