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, 2008 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 3, 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.
38 (defgroup calendar-dst nil
39 "Options related to Daylight Saving Time."
44 (defcustom calendar-dst-check-each-year-flag t
45 "Non-nil means to check each year for DST transitions as needed.
46 Otherwise assume the next two transitions found after the
47 current date apply to all years. This is faster, but not always
48 correct, since the dates of daylight saving transitions sometimes
55 (put 'calendar-daylight-savings-starts
'risky-local-variable t
)
56 (defcustom calendar-daylight-savings-starts
'(calendar-dst-starts year
)
57 "Sexp giving the date on which daylight saving time starts.
58 This is an expression in the variable `year' whose value gives the Gregorian
59 date in the form (month day year) on which daylight saving time starts. It is
60 used to determine the starting date of daylight saving time for the holiday
61 list and for correcting times of day in the solar and lunar calculations.
63 For example, if daylight saving time is mandated to start on October 1,
64 you would set `calendar-daylight-savings-starts' to
68 If it starts on the first Sunday in April, you would set it to
70 '(calendar-nth-named-day 1 0 4 year)
72 If the locale never uses daylight saving time, set this to nil."
77 (put 'calendar-daylight-savings-ends
'risky-local-variable t
)
78 (defcustom calendar-daylight-savings-ends
'(calendar-dst-ends year
)
79 "Sexp giving the date on which daylight saving time ends.
80 This is an expression in the variable `year' whose value gives the Gregorian
81 date in the form (month day year) on which daylight saving time ends. It is
82 used to determine the starting date of daylight saving time for the holiday
83 list and for correcting times of day in the solar and lunar calculations.
85 For example, if daylight saving time ends on the last Sunday in October:
87 '(calendar-nth-named-day -1 0 10 year)
89 If the locale never uses daylight saving time, set this to nil."
93 ;;; More defcustoms below.
96 (defvar calendar-current-time-zone-cache nil
97 "Cache for result of `calendar-current-time-zone'.")
98 ;; It gets eval'd, eg by calendar-dst-starts.
100 (put 'calendar-current-time-zone-cache
'risky-local-variable t
)
102 (defvar calendar-system-time-basis
103 (calendar-absolute-from-gregorian '(1 1 1970))
104 "Absolute date of starting date of system clock.")
106 (defun calendar-absolute-from-time (x utc-diff
)
107 "Absolute local date of time X; local time is UTC-DIFF seconds from UTC.
109 X is (HIGH . LOW) or (HIGH LOW . IGNORED) where HIGH and LOW are the
110 high and low 16 bits, respectively, of the number of seconds since
111 1970-01-01 00:00:00 UTC, ignoring leap seconds.
113 Returns the pair (ABS-DATE . SECONDS) where SECONDS after local midnight on
114 absolute date ABS-DATE is the equivalent moment to X."
117 (l (+ utc-diff
(if (numberp xtail
) xtail
(car xtail
))))
118 (u (+ (* 512 (mod h
675)) (floor l
128))))
119 ;; Overflow is a terrible thing!
120 (cons (+ calendar-system-time-basis
121 ;; floor((2^16 h +l) / (60*60*24))
122 (* 512 (floor h
675)) (floor u
675))
123 ;; (2^16 h +l) mod (60*60*24)
124 (+ (* (mod u
675) 128) (mod l
128)))))
126 (defun calendar-time-from-absolute (abs-date s
)
127 "Time of absolute date ABS-DATE, S seconds after midnight.
129 Returns the list (HIGH LOW) where HIGH and LOW are the high and low
130 16 bits, respectively, of the number of seconds 1970-01-01 00:00:00 UTC,
131 ignoring leap seconds, that is the equivalent moment to S seconds after
132 midnight UTC on absolute date ABS-DATE."
133 (let* ((a (- abs-date calendar-system-time-basis
))
134 (u (+ (* 163 (mod a
512)) (floor s
128))))
135 ;; Overflow is a terrible thing!
137 ;; floor((60*60*24*a + s) / 2^16)
138 (+ a
(* 163 (floor a
512)) (floor u
512))
139 ;; (60*60*24*a + s) mod 2^16
140 (+ (* 128 (mod u
512)) (mod s
128)))))
142 (defun calendar-next-time-zone-transition (time)
143 "Return the time of the next time zone transition after TIME.
144 Both TIME and the result are acceptable arguments to `current-time-zone'.
145 Return nil if no such transition can be found."
146 (let* ((base 65536) ; 2^16 = base of current-time output
147 (quarter-multiple 120) ; approx = (seconds per quarter year) / base
148 (time-zone (current-time-zone time
))
149 (time-utc-diff (car time-zone
))
152 (hi-utc-diff time-utc-diff
)
154 ;; Heuristic: probe the time zone offset in the next three calendar
155 ;; quarters, looking for a time zone offset different from TIME.
156 (while (and quarters
(eq time-utc-diff hi-utc-diff
))
157 (setq hi
(cons (+ (car time
) (* (car quarters
) quarter-multiple
)) 0)
158 hi-zone
(current-time-zone hi
)
159 hi-utc-diff
(car hi-zone
)
160 quarters
(cdr quarters
)))
164 (not (eq time-utc-diff hi-utc-diff
))
165 ;; Now HI is after the next time zone transition.
166 ;; Set LO to TIME, and then binary search to increase LO and decrease HI
167 ;; until LO is just before and HI is just after the time zone transition.
168 (let* ((tail (cdr time
))
169 (lo (cons (car time
) (if (numberp tail
) tail
(car tail
))))
172 ;; Set PROBE to halfway between LO and HI, rounding down.
173 ;; If PROBE equals LO, we are done.
174 (let* ((lsum (+ (cdr lo
) (cdr hi
)))
175 (hsum (+ (car lo
) (car hi
) (/ lsum base
)))
176 (hsumodd (logand 1 hsum
)))
177 (setq probe
(cons (/ (- hsum hsumodd
) 2)
178 (/ (+ (* hsumodd base
) (% lsum base
)) 2)))
179 (not (equal lo probe
)))
180 ;; Set either LO or HI to PROBE, depending on probe results.
181 (if (eq (car (current-time-zone probe
)) hi-utc-diff
)
186 (autoload 'calendar-persian-to-absolute
"cal-persia")
188 (defun calendar-time-zone-daylight-rules (abs-date utc-diff
)
189 "Return daylight transition rule for ABS-DATE, UTC-DIFF sec offset from UTC.
190 ABS-DATE must specify a day that contains a daylight saving transition.
191 The result has the proper form for `calendar-daylight-savings-starts'."
192 (let* ((date (calendar-gregorian-from-absolute abs-date
))
193 (weekday (% abs-date
7))
194 (m (calendar-extract-month date
))
195 (d (calendar-extract-day date
))
196 (y (calendar-extract-year date
))
197 (last (calendar-last-day-of-month m y
))
199 (candidate-rules ; these return Gregorian dates
203 ;; The first WEEKDAY of month M.
205 `((calendar-nth-named-day 1 ,weekday
,m year
)))
206 ;; The last WEEKDAY of month M.
208 `((calendar-nth-named-day -
1 ,weekday
,m year
)))
210 ;; The first WEEKDAY after day J of month M, for D-6 < J <= D.
211 (setq j
(1- (max 2 (- d
6))))
212 (while (<= (setq j
(1+ j
)) (min d
(- last
8)))
213 (push `(calendar-nth-named-day 1 ,weekday
,m year
,j
) rlist
))
215 ;; 01-01 and 07-01 for this year's Persian calendar.
216 ;; FIXME what does the Persian calendar have to do with this?
217 (if (and (= m
3) (<= 20 d
) (<= d
21))
218 '((calendar-gregorian-from-absolute
219 (calendar-persian-to-absolute `(1 1 ,(- year
621))))))
220 (if (and (= m
9) (<= 22 d
) (<= d
23))
221 '((calendar-gregorian-from-absolute
222 (calendar-persian-to-absolute `(7 1 ,(- year
621))))))))
223 (prevday-sec (- -
1 utc-diff
)) ; last sec of previous local day
226 ;; Scan through the next few years until only one rule remains.
227 (while (cdr candidate-rules
)
228 (dolist (rule candidate-rules
)
229 ;; The rule we return should give a Gregorian date, but here
230 ;; we require an absolute date. The following is for efficiency.
231 (setq date
(cond ((eq (car rule
) 'calendar-nth-named-day
)
232 (eval (cons 'calendar-nth-named-absday
(cdr rule
))))
233 ((eq (car rule
) 'calendar-gregorian-from-absolute
)
235 (t (calendar-absolute-from-gregorian (eval rule
)))))
236 (or (equal (current-time-zone
237 (calendar-time-from-absolute date prevday-sec
))
239 (calendar-time-from-absolute (1+ date
) prevday-sec
)))
240 (setq new-rules
(cons rule new-rules
))))
241 ;; If no rules remain, just use the first candidate rule;
242 ;; it's wrong in general, but it's right for at least one year.
243 (setq candidate-rules
(if new-rules
(nreverse new-rules
)
244 (list (car candidate-rules
)))
247 (car candidate-rules
)))
249 ;; TODO it might be better to extract this information directly from
250 ;; the system timezone database. But cross-platform...?
252 ;; http://lists.gnu.org/archive/html/emacs-pretest-bug/2006-11/msg00060.html
253 (defun calendar-dst-find-data (&optional time
)
254 "Find data on the first daylight saving time transitions after TIME.
255 TIME defaults to `current-time'. Return value is as described
256 for `calendar-current-time-zone'."
257 (let* ((t0 (or time
(current-time)))
258 (t0-zone (current-time-zone t0
))
259 (t0-utc-diff (car t0-zone
))
260 (t0-name (cadr t0-zone
)))
261 (if (not t0-utc-diff
)
262 ;; Little or no time zone information is available.
263 (list nil nil t0-name t0-name nil nil nil nil
)
264 (let* ((t1 (calendar-next-time-zone-transition t0
))
265 (t2 (and t1
(calendar-next-time-zone-transition t1
))))
267 ;; This locale does not have daylight saving time.
268 (list (/ t0-utc-diff
60) 0 t0-name t0-name nil nil
0 0)
269 ;; Use heuristics to find daylight saving parameters.
270 (let* ((t1-zone (current-time-zone t1
))
271 (t1-utc-diff (car t1-zone
))
272 (t1-name (cadr t1-zone
))
273 (t1-date-sec (calendar-absolute-from-time t1 t0-utc-diff
))
274 (t2-date-sec (calendar-absolute-from-time t2 t1-utc-diff
))
275 ;; TODO When calendar-dst-check-each-year-flag is non-nil,
276 ;; the rules can be simpler than they currently are.
277 (t1-rules (calendar-time-zone-daylight-rules
278 (car t1-date-sec
) t0-utc-diff
))
279 (t2-rules (calendar-time-zone-daylight-rules
280 (car t2-date-sec
) t1-utc-diff
))
281 (t1-time (/ (cdr t1-date-sec
) 60))
282 (t2-time (/ (cdr t2-date-sec
) 60)))
284 (/ (min t0-utc-diff t1-utc-diff
) 60)
286 (/ (abs (- t0-utc-diff t1-utc-diff
)) 60)
287 (if (< t0-utc-diff t1-utc-diff
)
288 (list t0-name t1-name t1-rules t2-rules t1-time t2-time
)
289 (list t1-name t0-name t2-rules t1-rules t2-time t1-time
)
292 (defvar calendar-dst-transition-cache nil
293 "Internal cal-dst variable storing date of daylight saving time transitions.
294 Value is a list with elements of the form (YEAR START END), where
295 START and END are expressions that when evaluated return the
296 start and end dates (respectively) for DST in YEAR. Used by the
297 function `calendar-dst-find-startend'.")
299 (defun calendar-dst-find-startend (year)
300 "Find the dates in YEAR on which daylight saving time starts and ends.
301 Returns a list (YEAR START END), where START and END are
302 expressions that when evaluated return the start and end dates,
303 respectively. This function first attempts to use pre-calculated
304 data from `calendar-dst-transition-cache', otherwise it calls
305 `calendar-dst-find-data' (and adds the results to the cache).
306 If dates in YEAR cannot be handled by `encode-time' (e.g. if they
307 are too large to be represented as a lisp integer), then rather
308 than an error this function returns the result appropriate for
310 (let ((e (assoc year calendar-dst-transition-cache
))
314 (setq e
(calendar-dst-find-data
316 (encode-time 1 0 0 1 1 year
)
318 (encode-time 1 0 0 1 1 (nth 5 (decode-time))))))
320 e
(list year f
(nth 5 e
))
321 calendar-dst-transition-cache
322 (append calendar-dst-transition-cache
(list e
)))
325 (defun calendar-current-time-zone ()
326 "Return UTC difference, dst offset, names and rules for current time zone.
328 Returns (UTC-DIFF DST-OFFSET STD-ZONE DST-ZONE DST-STARTS DST-ENDS
329 DST-STARTS-TIME DST-ENDS-TIME), based on a heuristic probing of what the
332 UTC-DIFF is an integer specifying the number of minutes difference between
333 standard time in the current time zone and Coordinated Universal Time
334 (Greenwich Mean Time). A negative value means west of Greenwich.
335 DST-OFFSET is an integer giving the daylight saving time offset in minutes.
336 STD-ZONE is a string giving the name of the time zone when no seasonal time
337 adjustment is in effect.
338 DST-ZONE is a string giving the name of the time zone when there is a seasonal
339 time adjustment in effect.
340 DST-STARTS and DST-ENDS are sexps in the variable `year' giving the daylight
341 saving time start and end rules, in the form expected by
342 `calendar-daylight-savings-starts'.
343 DST-STARTS-TIME and DST-ENDS-TIME are integers giving the number of minutes
344 after midnight that daylight saving time starts and ends.
346 If the local area does not use a seasonal time adjustment, STD-ZONE and
347 DST-ZONE are equal, and all the DST-* integer variables are 0.
349 Some operating systems cannot provide all this information to Emacs; in this
350 case, `calendar-current-time-zone' returns a list containing nil for the data
352 (unless calendar-current-time-zone-cache
353 (setq calendar-current-time-zone-cache
(calendar-dst-find-data))))
356 ;; Following options should be set based on conditions when the code
357 ;; is invoked, so are not suitable for dumping into loaddefs.el. They
358 ;; default to US Eastern time if time zone info is not available.
360 (calendar-current-time-zone)
362 (defcustom calendar-time-zone
(or (car calendar-current-time-zone-cache
) -
300)
363 "Number of minutes difference between local standard time and UTC.
364 For example, -300 for New York City, -480 for Los Angeles."
366 :group
'calendar-dst
)
368 (defcustom calendar-daylight-time-offset
369 (or (cadr calendar-current-time-zone-cache
) 60)
370 "Number of minutes difference between daylight saving and standard time.
371 If the locale never uses daylight saving time, set this to 0."
373 :group
'calendar-dst
)
375 (defcustom calendar-standard-time-zone-name
376 (or (nth 2 calendar-current-time-zone-cache
) "EST")
377 "Abbreviated name of standard time zone at `calendar-location-name'.
378 For example, \"EST\" in New York City, \"PST\" for Los Angeles."
380 :group
'calendar-dst
)
382 (defcustom calendar-daylight-time-zone-name
383 (or (nth 3 calendar-current-time-zone-cache
) "EDT")
384 "Abbreviated name of daylight saving time zone at `calendar-location-name'.
385 For example, \"EDT\" in New York City, \"PDT\" for Los Angeles."
387 :group
'calendar-dst
)
389 (defcustom calendar-daylight-savings-starts-time
390 (or (nth 6 calendar-current-time-zone-cache
) 120)
391 "Number of minutes after midnight that daylight saving time starts."
393 :group
'calendar-dst
)
395 (defcustom calendar-daylight-savings-ends-time
396 (or (nth 7 calendar-current-time-zone-cache
)
397 calendar-daylight-savings-starts-time
)
398 "Number of minutes after midnight that daylight saving time ends."
400 :group
'calendar-dst
)
403 (defun calendar-dst-starts (year)
404 "Return the date of YEAR on which daylight saving time starts.
405 This function respects the value of `calendar-dst-check-each-year-flag'."
406 (or (let ((expr (if calendar-dst-check-each-year-flag
407 (cadr (calendar-dst-find-startend year
))
408 (nth 4 calendar-current-time-zone-cache
))))
409 (if expr
(eval expr
)))
410 ;; New US rules commencing 2007. ftp://elsie.nci.nih.gov/pub/.
411 (and (not (zerop calendar-daylight-time-offset
))
412 (calendar-nth-named-day 2 0 3 year
))))
414 (defun calendar-dst-ends (year)
415 "Return the date of YEAR on which daylight saving time ends.
416 This function respects the value of `calendar-dst-check-each-year-flag'."
417 (or (let ((expr (if calendar-dst-check-each-year-flag
418 (nth 2 (calendar-dst-find-startend year
))
419 (nth 5 calendar-current-time-zone-cache
))))
420 (if expr
(eval expr
)))
421 ;; New US rules commencing 2007. ftp://elsie.nci.nih.gov/pub/.
422 (and (not (zerop calendar-daylight-time-offset
))
423 (calendar-nth-named-day 1 0 11 year
))))
425 ;; used by calc, solar.
426 (defun dst-in-effect (date)
427 "True if on absolute DATE daylight saving time is in effect.
428 Fractional part of DATE is local standard time of day."
429 (let* ((year (calendar-extract-year
430 (calendar-gregorian-from-absolute (floor date
))))
431 (dst-starts-gregorian (eval calendar-daylight-savings-starts
))
432 (dst-ends-gregorian (eval calendar-daylight-savings-ends
))
433 (dst-starts (and dst-starts-gregorian
434 (+ (calendar-absolute-from-gregorian
435 dst-starts-gregorian
)
436 (/ calendar-daylight-savings-starts-time
438 (dst-ends (and dst-ends-gregorian
439 (+ (calendar-absolute-from-gregorian
441 (/ (- calendar-daylight-savings-ends-time
442 calendar-daylight-time-offset
)
444 (and dst-starts dst-ends
445 (if (< dst-starts dst-ends
)
446 (and (<= dst-starts date
) (< date dst-ends
))
447 (or (<= dst-starts date
) (< date dst-ends
))))))
449 ;; used by calc, lunar, solar.
450 (defun dst-adjust-time (date time
&optional style
)
451 "Adjust, to account for dst on DATE, decimal fraction standard TIME.
452 Returns a list (date adj-time zone) where `date' and `adj-time' are the values
453 adjusted for `zone'; here `date' is a list (month day year), `adj-time' is a
454 decimal fraction time, and `zone' is a string.
456 Optional parameter STYLE forces the result time to be standard time when its
457 value is 'standard and daylight saving time (if available) when its value is
460 Conversion to daylight saving time is done according to
461 `calendar-daylight-savings-starts', `calendar-daylight-savings-ends',
462 `calendar-daylight-savings-starts-time',
463 `calendar-daylight-savings-ends-time', and `calendar-daylight-time-offset'."
464 (let* ((rounded-abs-date (+ (calendar-absolute-from-gregorian date
)
465 (/ (round (* 60 time
)) 60.0 24.0)))
466 (dst (dst-in-effect rounded-abs-date
))
468 calendar-daylight-time-zone-name
469 calendar-standard-time-zone-name
))
470 (time (+ rounded-abs-date
471 (if dst
(/ calendar-daylight-time-offset
24.0 60.0) 0))))
472 (list (calendar-gregorian-from-absolute (truncate time
))
473 (* 24.0 (- time
(truncate time
)))
478 ;; arch-tag: a141d204-213c-4ca5-bdc6-f9df3aa92aad
479 ;;; cal-dst.el ends here