1 ;;; cal-dst.el --- calendar functions for daylight savings rules.
3 ;; Copyright (C) 1993 Free Software Foundation, Inc.
5 ;; Author: Paul Eggert <eggert@twinsun.com>
6 ;; Edward M. Reingold <reingold@cs.uiuc.edu>
8 ;; Human-Keywords: daylight savings time, calendar, diary, holidays
10 ;; This file is part of GNU Emacs.
12 ;; GNU Emacs is distributed in the hope that it will be useful,
13 ;; but WITHOUT ANY WARRANTY. No author or distributor
14 ;; accepts responsibility to anyone for the consequences of using it
15 ;; or for whether it serves any particular purpose or works at all,
16 ;; unless he says so in writing. Refer to the GNU Emacs General Public
17 ;; License for full details.
19 ;; Everyone is granted permission to copy, modify and redistribute
20 ;; GNU Emacs, but only under the conditions described in the
21 ;; GNU Emacs General Public License. A copy of this license is
22 ;; supposed to have been given to you along with GNU Emacs so you
23 ;; can know your rights and responsibilities. It should be in a
24 ;; file named COPYING. Among other things, the copyright notice
25 ;; and this notice must be preserved on all copies.
29 ;; This collection of functions implements the features of calendar.el and
30 ;; holiday.el that deal with daylight savings time.
32 ;; Comments, corrections, and improvements should be sent to
33 ;; Edward M. Reingold Department of Computer Science
34 ;; (217) 333-6733 University of Illinois at Urbana-Champaign
35 ;; reingold@cs.uiuc.edu 1304 West Springfield Avenue
36 ;; Urbana, Illinois 61801
42 (defvar calendar-current-time-zone-cache nil
43 "Cache for result of calendar-current-time-zone.")
45 (defvar calendar-system-time-basis
46 (calendar-absolute-from-gregorian '(1 1 1970))
47 "Absolute date of starting date of system clock.")
49 (defun calendar-absolute-from-time (x utc-diff)
50 "Absolute local date of time X; local time is UTC-DIFF seconds from UTC.
52 X is (HIGH . LOW) or (HIGH LOW . IGNORED) where HIGH and LOW are the
53 high and low 16 bits, respectively, of the number of seconds since
54 1970-01-01 00:00:00 UTC, ignoring leap seconds.
56 Returns the pair (ABS-DATE . SECONDS) where SECONDS after local midnight on
57 absolute date ABS-DATE is the equivalent moment to X."
60 (l (+ utc-diff (if (numberp xtail) xtail (car xtail))))
61 (u (+ (* 512 (mod h 675)) (floor l 128))))
62 ;; Overflow is a terrible thing!
63 (cons (+ calendar-system-time-basis
64 ;; floor((2^16 h +l) / (60*60*24))
65 (* 512 (floor h 675)) (floor u 675))
66 ;; (2^16 h +l) % (60*60*24)
67 (+ (* (mod u 675) 128) (mod l 128)))))
69 (defun calendar-time-from-absolute (abs-date s)
70 "Time of absolute date ABS-DATE, S seconds after midnight.
72 Returns the pair (HIGH . LOW) where HIGH and LOW are the high and low
73 16 bits, respectively, of the number of seconds 1970-01-01 00:00:00 UTC,
74 ignoring leap seconds, that is the equivalent moment to S seconds after
75 midnight UTC on absolute date ABS-DATE."
76 (let* ((a (- abs-date calendar-system-time-basis))
77 (u (+ (* 163 (mod a 512)) (floor s 128))))
78 ;; Overflow is a terrible thing!
80 ;; (60*60*24*a + s) / 2^16
81 (+ a (* 163 (floor a 512)) (floor u 512))
82 ;; (60*60*24*a + s) % 2^16
83 (+ (* 128 (mod u 512)) (mod s 128)))))
85 (defun calendar-next-time-zone-transition (time)
86 "Return the time of the next time zone transition after TIME.
87 Both TIME and the result are acceptable arguments to current-time-zone.
88 Return nil if no such transition can be found."
89 (let* ((base 65536);; 2^16 = base of current-time output
90 (quarter-multiple 120);; approx = (seconds per quarter year) / base
91 (time-zone (current-time-zone time))
92 (time-utc-diff (car time-zone))
95 (hi-utc-diff time-utc-diff)
97 ;; Heuristic: probe the time zone offset in the next three calendar
98 ;; quarters, looking for a time zone offset different from TIME.
99 (while (and quarters (eq time-utc-diff hi-utc-diff))
100 (setq hi (cons (+ (car time) (* (car quarters) quarter-multiple)) 0))
101 (setq hi-zone (current-time-zone hi))
102 (setq hi-utc-diff (car hi-zone))
103 (setq quarters (cdr quarters)))
107 (not (eq time-utc-diff hi-utc-diff))
108 ;; Now HI is after the next time zone transition.
109 ;; Set LO to TIME, and then binary search to increase LO and decrease HI
110 ;; until LO is just before and HI is just after the time zone transition.
111 (let* ((tail (cdr time))
112 (lo (cons (car time) (if (numberp tail) tail (car tail))))
115 ;; Set PROBE to halfway between LO and HI, rounding down.
116 ;; If PROBE equals LO, we are done.
117 (let* ((lsum (+ (cdr lo) (cdr hi)))
118 (hsum (+ (car lo) (car hi) (/ lsum base)))
119 (hsumodd (logand 1 hsum)))
120 (setq probe (cons (/ (- hsum hsumodd) 2)
121 (/ (+ (* hsumodd base) (% lsum base)) 2)))
122 (not (equal lo probe)))
123 ;; Set either LO or HI to PROBE, depending on probe results.
124 (if (eq (car (current-time-zone probe)) hi-utc-diff)
129 (defun calendar-time-zone-daylight-rules (abs-date utc-diff)
130 "Return daylight transition rule for ABS-DATE, UTC-DIFF sec offset from UTC.
131 ABS-DIFF must specify a day that contains a daylight savings transition.
132 The result has the proper form for calendar-daylight-savings-starts'."
133 (let* ((date (calendar-gregorian-from-absolute abs-date))
134 (weekday (% abs-date 7))
135 (m (extract-calendar-month date))
136 (d (extract-calendar-day date))
137 (y (extract-calendar-year date))
138 (last (calendar-last-day-of-month m y))
142 (list (list 'list m d 'year))
143 ;; The first WEEKDAY of month M.
145 (list (list 'calendar-nth-named-day 1 weekday m 'year)))
146 ;; The last WEEKDAY of month M.
148 (list (list 'calendar-nth-named-day -1 weekday m 'year)))
149 ;; The first WEEKDAY after day J of month M, for D-6 < J <= D.
151 (calendar-for-loop j from (max 2 (- d 6)) to (min d (- last 8)) do
154 (list 'calendar-nth-named-day 1 weekday m 'year j)
157 ;; Israel is special.
161 '(calendar-gregorian-from-absolute
162 (calendar-dayname-on-or-before
164 (calendar-absolute-from-hebrew
165 (list 1 28 (+ year 3760))))))
166 (list '(calendar-gregorian-from-absolute
167 (calendar-dayname-on-or-before
169 (- (calendar-absolute-from-hebrew
170 (list 7 1 (+ year 3761))) 3))))))))
171 (prevday-sec (- -1 utc-diff)) ;; last sec of previous local day
173 ;; Scan through the next few years until only one rule remains.
175 (let ((rules candidate-rules)
181 ;; The following is much faster than
182 ;; (calendar-absolute-from-gregorian (eval rule)).
183 (cond ((eq (car rule) 'calendar-nth-named-day)
184 (eval (cons 'calendar-nth-named-absday (cdr rule))))
185 ((eq (car rule) 'calendar-gregorian-from-absolute)
186 (eval (car (cdr rule))))
187 (t (let ((g (eval rule)))
188 (calendar-absolute-from-gregorian g))))))
191 (calendar-time-from-absolute date prevday-sec))
193 (calendar-time-from-absolute (1+ date) prevday-sec)))
194 (setq new-rules (cons rule new-rules)))
195 (setq rules (cdr rules))))
196 ;; If no rules remain, just use the first candidate rule;
197 ;; it's wrong in general, but it's right for at least one year.
198 (setq candidate-rules (if new-rules (nreverse new-rules)
199 (list (car candidate-rules))))
200 (setq year (1+ year))
201 (cdr candidate-rules)))
202 (car candidate-rules)))
204 (defun calendar-current-time-zone ()
205 "Return UTC difference, dst offset, names and rules for current time zone.
207 Returns (UTC-DIFF DST-OFFSET STD-ZONE DST-ZONE DST-STARTS DST-ENDS
208 DST-STARTS-TIME DST-ENDS-TIME), based on a heuristic probing of what the
211 UTC-DIFF is an integer specifying the number of minutes difference between
212 standard time in the current time zone and Coordinated Universal Time
213 (Greenwich Mean Time). A negative value means west of Greenwich.
214 DST-OFFSET is an integer giving the daylight savings time offset in minutes.
215 STD-ZONE is a string giving the name of the time zone when no seasonal time
216 adjustment is in effect.
217 DST-ZONE is a string giving the name of the time zone when there is a seasonal
218 time adjustment in effect.
219 DST-STARTS and DST-ENDS are sexps in the variable `year' giving the daylight
220 savings time start and end rules, in the form expected by
221 `calendar-daylight-savings-starts'.
222 DST-STARTS-TIME and DST-ENDS-TIME are integers giving the number of minutes
223 after midnight that daylight savings time starts and ends.
225 If the local area does not use a seasonal time adjustment, STD-ZONE and
226 DST-ZONE are equal, and all the DST-* integer variables are 0.
228 Some operating systems cannot provide all this information to Emacs; in this
229 case, `calendar-current-time-zone' returns a list containing nil for the data
232 calendar-current-time-zone-cache
234 calendar-current-time-zone-cache
235 (let* ((t0 (current-time))
236 (t0-zone (current-time-zone t0))
237 (t0-utc-diff (car t0-zone))
238 (t0-name (car (cdr t0-zone))))
239 (if (not t0-utc-diff)
240 ;; Little or no time zone information is available.
241 (list nil nil t0-name t0-name nil nil nil nil)
242 (let* ((t1 (calendar-next-time-zone-transition t0))
243 (t2 (and t1 (calendar-next-time-zone-transition t1))))
245 ;; This locale does not have daylight savings time.
246 (list (/ t0-utc-diff 60) 0 t0-name t0-name nil nil 0 0)
247 ;; Use heuristics to find daylight savings parameters.
248 (let* ((t1-zone (current-time-zone t1))
249 (t1-utc-diff (car t1-zone))
250 (t1-name (car (cdr t1-zone)))
251 (t1-date-sec (calendar-absolute-from-time t1 t0-utc-diff))
252 (t2-date-sec (calendar-absolute-from-time t2 t1-utc-diff))
253 (t1-rules (calendar-time-zone-daylight-rules
254 (car t1-date-sec) t0-utc-diff))
255 (t2-rules (calendar-time-zone-daylight-rules
256 (car t2-date-sec) t1-utc-diff))
257 (t1-time (/ (cdr t1-date-sec) 60))
258 (t2-time (/ (cdr t2-date-sec) 60)))
260 (/ (min t0-utc-diff t1-utc-diff) 60)
262 (/ (abs (- t0-utc-diff t1-utc-diff)) 60)
263 (if (< t0-utc-diff t1-utc-diff)
264 (list t0-name t1-name t1-rules t2-rules t2-time t1-time)
265 (list t1-name t0-name t2-rules t1-rules t1-time t2-time)
268 ;;; The following six defvars relating to daylight savings time should NOT be
269 ;;; marked to go into loaddefs.el where they would be evaluated when Emacs is
270 ;;; dumped. These variables' appropriate values depend on the conditions under
271 ;;; which the code is INVOKED; so it's inappropriate to initialize them when
272 ;;; Emacs is dumped---they should be initialized when calendar.el is loaded.
273 ;;; They default to US Eastern time if time zone info is not available.
275 (calendar-current-time-zone)
277 (defvar calendar-time-zone (or (car calendar-current-time-zone-cache) -300)
278 "*Number of minutes difference between local standard time at
279 `calendar-location-name' and Coordinated Universal (Greenwich) Time. For
280 example, -300 for New York City, -480 for Los Angeles.")
282 (defvar calendar-daylight-time-offset
283 (or (car (cdr calendar-current-time-zone-cache)) 60)
284 "*Number of minutes difference between daylight savings and standard time.
286 If the locale never uses daylight savings time, set this to 0.")
288 (defvar calendar-standard-time-zone-name
289 (or (car (nthcdr 2 calendar-current-time-zone-cache)) "EST")
290 "*Abbreviated name of standard time zone at `calendar-location-name'.
291 For example, \"EST\" in New York City, \"PST\" for Los Angeles.")
293 (defvar calendar-daylight-time-zone-name
294 (or (car (nthcdr 3 calendar-current-time-zone-cache)) "EDT")
295 "*Abbreviated name of daylight-savings time zone at `calendar-location-name'.
296 For example, \"EDT\" in New York City, \"PDT\" for Los Angeles.")
298 (defvar calendar-daylight-savings-starts
299 (or (car (nthcdr 4 calendar-current-time-zone-cache))
300 (and (not (zerop calendar-daylight-time-offset))
301 '(calendar-nth-named-day 1 0 4 year)))
302 "*Sexp giving the date on which daylight savings time starts.
303 This is an expression in the variable `year' whose value gives the Gregorian
304 date in the form (month day year) on which daylight savings time starts. It is
305 used to determine the starting date of daylight savings time for the holiday
306 list and for correcting times of day in the solar and lunar calculations.
308 For example, if daylight savings time is mandated to start on October 1,
309 you would set `calendar-daylight-savings-starts' to
313 For a more complex example, daylight savings time begins in Israel on the
314 first Sunday after Passover ends on Nisan 21:
316 '(calendar-gregorian-from-absolute
317 (calendar-dayname-on-or-before
319 (calendar-absolute-from-hebrew (list 1 28 (+ year 3760)))))
321 because Nisan is the first month in the Hebrew calendar.
323 If the locale never uses daylight savings time, set this to nil.")
325 (defvar calendar-daylight-savings-ends
326 (or (car (nthcdr 5 calendar-current-time-zone-cache))
327 (and (not (zerop calendar-daylight-time-offset))
328 '(calendar-nth-named-day -1 0 10 year)))
329 "*Sexp giving the date on which daylight savings time ends.
330 This is an expression in the variable `year' whose value gives the Gregorian
331 date in the form (month day year) on which daylight savings time ends. It is
332 used to determine the starting date of daylight savings time for the holiday
333 list and for correcting times of day in the solar and lunar calculations.
335 For example, daylight savings time ends in Israel on the Sunday Selichot
338 '(calendar-gregorian-from-absolute
339 (calendar-dayname-on-or-before
341 (- (calendar-absolute-from-hebrew (list 7 1 (+ year 3761))) 3)))
343 If the locale never uses daylight savings time, set this to nil.")
345 (defvar calendar-daylight-savings-starts-time
346 (or (car (nthcdr 6 calendar-current-time-zone-cache)) 120)
347 "*Number of minutes after midnight that daylight savings time starts.")
349 (defvar calendar-daylight-savings-ends-time
350 (or (car (nthcdr 7 calendar-current-time-zone-cache))
351 calendar-daylight-savings-starts-time)
352 "*Number of minutes after midnight that daylight savings time ends.")
356 ;;; cal-dst.el ends here