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 (mod h 675)) (floor u 675))
66 ;; (2^16 h +l) % (60*60*24)
67 (+ (* (mod u 675) 128) (floor 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
174 ;; Scan through the next few years; take the rule that explains them best.
175 (while (and candidate-rules (cdr candidate-rules) (<= i 28))
178 (while candidate-rules
179 (let* ((rule (car candidate-rules))
180 (date (calendar-absolute-from-gregorian (eval rule))))
181 (or (equal (current-time-zone
182 (calendar-time-from-absolute date prevday-sec))
184 (calendar-time-from-absolute (1+ date) prevday-sec)))
186 (setq new-rules (cons rule new-rules))
187 (setq last-surviving-rule rule))))
188 (setq candidate-rules (cdr candidate-rules)))
189 (setq candidate-rules (nreverse new-rules)))
191 last-surviving-rule))
193 (defun calendar-current-time-zone ()
194 "Return UTC difference, dst offset, names and rules for current time zone.
196 Returns (UTC-DIFF DST-OFFSET STD-ZONE DST-ZONE DST-STARTS DST-ENDS DST-SWITCH),
197 based on a heuristic probing of what the system knows:
199 UTC-DIFF is an integer specifying the number of minutes difference between
200 standard time in the current time zone and Coordinated Universal Time
201 (Greenwich Mean Time). A negative value means west of Greenwich.
202 DST-OFFSET is an integer giving the daylight savings time offset in minutes.
203 STD-ZONE is a string giving the name of the time zone when no seasonal time
204 adjustment is in effect.
205 DST-ZONE is a string giving the name of the time zone when there is a seasonal
206 time adjustment in effect.
207 DST-STARTS and DST-ENDS are sexps in the variable `year' giving the daylight
208 savings time start rules, in the form expected by
209 `calendar-daylight-savings-starts'.
210 DST-SWITCH is an integer giving the number of minutes after midnight that
211 daylight savings time starts or ends.
213 If the local area does not use a seasonal time adjustment, DST-OFFSET and
214 DST-SWITCH are 0, STD-ZONE and DST-ZONE are equal, and DST-STARTS and DST-ENDS
217 Some operating systems cannot provide all this information to Emacs; in this
218 case, `calendar-current-time-zone' returns a list containing nil for the data
221 calendar-current-time-zone-cache
223 (message "Checking time zone data...")
225 calendar-current-time-zone-cache
226 (let* ((now (current-time))
227 (now-zone (current-time-zone now))
228 (now-utc-diff (car now-zone))
229 (now-name (car (cdr now-zone)))
230 (next (calendar-next-time-zone-transition now)))
232 (list (and now-utc-diff (/ now-utc-diff 60))
233 0 now-name now-name nil nil 0)
234 (let* ((next-zone (current-time-zone next))
235 (next-utc-diff (car next-zone))
236 (next-name (car (cdr next-zone)))
237 (next-absdate-seconds
238 (calendar-absolute-from-time next now-utc-diff))
240 (calendar-time-zone-daylight-rules
241 (car next-absdate-seconds) now-utc-diff))
242 (nextnext (calendar-next-time-zone-transition next))
244 (calendar-time-zone-daylight-rules
245 (car (calendar-absolute-from-time nextnext next-utc-diff))
247 (now-is-std (< now-utc-diff next-utc-diff)))
248 (list (/ (min now-utc-diff next-utc-diff) 60)
249 (/ (abs (- now-utc-diff next-utc-diff)) 60)
250 (if now-is-std now-name next-name)
251 (if now-is-std next-name now-name)
252 (if now-is-std next-transitions now-transitions)
253 (if now-is-std now-transitions next-transitions)
254 (/ (cdr next-absdate-seconds) 60))))))
255 (message "Checking time zone data...done")))
256 calendar-current-time-zone-cache)
258 ;;; The following six defvars relating to daylight savings time should NOT be
259 ;;; marked to go into loaddefs.el where they would be evaluated when Emacs is
260 ;;; dumped. These variables' appropriate values depend on the conditions under
261 ;;; which the code is INVOKED; so it's inappropriate to initialize them when
262 ;;; Emacs is dumped---they should be initialized when calendar.el is loaded.
264 (calendar-current-time-zone)
266 (defvar calendar-time-zone (car calendar-current-time-zone-cache)
267 "*Number of minutes difference between local standard time at
268 `calendar-location-name' and Coordinated Universal (Greenwich) Time. For
269 example, -300 for New York City, -480 for Los Angeles.")
271 (defvar calendar-daylight-time-offset
272 (car (cdr calendar-current-time-zone-cache))
273 "*Number of minutes difference between daylight savings and standard time.
275 If the locale never uses daylight savings time, set this to 0.")
277 (defvar calendar-standard-time-zone-name
278 (car (nthcdr 2 calendar-current-time-zone-cache))
279 "*Abbreviated name of standard time zone at `calendar-location-name'.
280 For example, \"EST\" in New York City, \"PST\" for Los Angeles.")
282 (defvar calendar-daylight-time-zone-name
283 (car (nthcdr 3 calendar-current-time-zone-cache))
284 "*Abbreviated name of daylight-savings time zone at `calendar-location-name'.
285 For example, \"EDT\" in New York City, \"PDT\" for Los Angeles.")
287 (defvar calendar-daylight-savings-starts
288 (car (nthcdr 4 calendar-current-time-zone-cache))
289 "*Sexp giving the date on which daylight savings time starts.
290 This is an expression in the variable `year' whose value gives the Gregorian
291 date in the form (month day year) on which daylight savings time starts. It is
292 used to determine the starting date of daylight savings time for the holiday
293 list and for correcting times of day in the solar and lunar calculations.
295 For example, if daylight savings time is mandated to start on October 1,
296 you would set `calendar-daylight-savings-starts' to
300 For a more complex example, daylight savings time begins in Israel on the
301 first Sunday after Passover ends on Nisan 21:
303 '(calendar-gregorian-from-absolute
304 (calendar-dayname-on-or-before
306 (calendar-absolute-from-hebrew (list 1 28 (+ year 3760)))))
308 because Nisan is the first month in the Hebrew calendar.
310 If the locale never uses daylight savings time, set this to nil.")
312 (defvar calendar-daylight-savings-ends
313 (car (nthcdr 5 calendar-current-time-zone-cache))
314 "*Sexp giving the date on which daylight savings time ends.
315 This is an expression in the variable `year' whose value gives the Gregorian
316 date in the form (month day year) on which daylight savings time ends. It is
317 used to determine the starting date of daylight savings time for the holiday
318 list and for correcting times of day in the solar and lunar calculations.
320 For example, daylight savings time ends in Israel on the Sunday Selichot
323 '(calendar-gregorian-from-absolute
324 (calendar-dayname-on-or-before
326 (- (calendar-absolute-from-hebrew (list 7 1 (+ year 3761))) 3)))
328 If the locale never uses daylight savings time, set this to nil.")
330 (defvar calendar-daylight-savings-switchover-time
331 (car (nthcdr 6 calendar-current-time-zone-cache))
332 "*Number of minutes after midnight that daylight savings time begins/ends.
333 If the locale never uses daylight savings time, set this to 0.")
337 ;;; cal-dst.el ends here