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1 ;;; cal-dst.el --- calendar functions for daylight savings rules.
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2
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3 ;; Copyright (C) 1993, 1994 Free Software Foundation, Inc.
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4
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5 ;; Author: Paul Eggert <eggert@twinsun.com>
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6 ;; Edward M. Reingold <reingold@cs.uiuc.edu>
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7 ;; Keywords: calendar
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8 ;; Human-Keywords: daylight savings time, calendar, diary, holidays
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9
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10 ;; This file is part of XEmacs.
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11
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12 ;; XEmacs is free software; you can redistribute it and/or modify it
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13 ;; under the terms of the GNU General Public License as published by
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14 ;; the Free Software Foundation; either version 2, or (at your option)
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15 ;; any later version.
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16
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17 ;; XEmacs is distributed in the hope that it will be useful, but
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18 ;; WITHOUT ANY WARRANTY; without even the implied warranty of
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19 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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20 ;; General Public License for more details.
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21
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22 ;; You should have received a copy of the GNU General Public License
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23 ;; along with XEmacs; see the file COPYING. If not, write to the Free
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24 ;; Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
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25
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26 ;;; Commentary:
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27
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28 ;; This collection of functions implements the features of calendar.el and
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29 ;; holiday.el that deal with daylight savings time.
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30
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31 ;; Comments, corrections, and improvements should be sent to
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32 ;; Edward M. Reingold Department of Computer Science
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33 ;; (217) 333-6733 University of Illinois at Urbana-Champaign
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34 ;; reingold@cs.uiuc.edu 1304 West Springfield Avenue
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35 ;; Urbana, Illinois 61801
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36
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37 ;;; Code:
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38
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39 (require 'calendar)
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40
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41 (defvar calendar-current-time-zone-cache nil
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42 "Cache for result of calendar-current-time-zone.")
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43
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44 (defvar calendar-system-time-basis
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45 (calendar-absolute-from-gregorian '(1 1 1970))
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46 "Absolute date of starting date of system clock.")
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47
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48 (defun calendar-absolute-from-time (x utc-diff)
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49 "Absolute local date of time X; local time is UTC-DIFF seconds from UTC.
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50
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51 X is (HIGH . LOW) or (HIGH LOW . IGNORED) where HIGH and LOW are the
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52 high and low 16 bits, respectively, of the number of seconds since
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53 1970-01-01 00:00:00 UTC, ignoring leap seconds.
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54
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55 Returns the pair (ABS-DATE . SECONDS) where SECONDS after local midnight on
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56 absolute date ABS-DATE is the equivalent moment to X."
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57 (let* ((h (car x))
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58 (xtail (cdr x))
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59 (l (+ utc-diff (if (numberp xtail) xtail (car xtail))))
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60 (u (+ (* 512 (mod h 675)) (floor l 128))))
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61 ;; Overflow is a terrible thing!
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62 (cons (+ calendar-system-time-basis
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63 ;; floor((2^16 h +l) / (60*60*24))
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64 (* 512 (floor h 675)) (floor u 675))
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65 ;; (2^16 h +l) mod (60*60*24)
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66 (+ (* (mod u 675) 128) (mod l 128)))))
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67
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68 (defun calendar-time-from-absolute (abs-date s)
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69 "Time of absolute date ABS-DATE, S seconds after midnight.
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70
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71 Returns the pair (HIGH . LOW) where HIGH and LOW are the high and low
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72 16 bits, respectively, of the number of seconds 1970-01-01 00:00:00 UTC,
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73 ignoring leap seconds, that is the equivalent moment to S seconds after
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74 midnight UTC on absolute date ABS-DATE."
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75 (let* ((a (- abs-date calendar-system-time-basis))
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76 (u (+ (* 163 (mod a 512)) (floor s 128))))
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77 ;; Overflow is a terrible thing!
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78 (cons
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79 ;; floor((60*60*24*a + s) / 2^16)
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80 (+ a (* 163 (floor a 512)) (floor u 512))
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81 ;; (60*60*24*a + s) mod 2^16
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82 (+ (* 128 (mod u 512)) (mod s 128)))))
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83
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84 (defun calendar-next-time-zone-transition (time)
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85 "Return the time of the next time zone transition after TIME.
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86 Both TIME and the result are acceptable arguments to current-time-zone.
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87 Return nil if no such transition can be found."
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88 (let* ((base 65536);; 2^16 = base of current-time output
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89 (quarter-multiple 120);; approx = (seconds per quarter year) / base
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90 (time-zone (current-time-zone time))
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91 (time-utc-diff (car time-zone))
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92 hi
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93 hi-zone
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94 (hi-utc-diff time-utc-diff)
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95 (quarters '(2 1 3)))
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96 ;; Heuristic: probe the time zone offset in the next three calendar
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97 ;; quarters, looking for a time zone offset different from TIME.
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98 (while (and quarters (eq time-utc-diff hi-utc-diff))
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99 (setq hi (cons (+ (car time) (* (car quarters) quarter-multiple)) 0))
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100 (setq hi-zone (current-time-zone hi))
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101 (setq hi-utc-diff (car hi-zone))
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102 (setq quarters (cdr quarters)))
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103 (and
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104 time-utc-diff
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105 hi-utc-diff
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106 (not (eq time-utc-diff hi-utc-diff))
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107 ;; Now HI is after the next time zone transition.
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108 ;; Set LO to TIME, and then binary search to increase LO and decrease HI
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109 ;; until LO is just before and HI is just after the time zone transition.
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110 (let* ((tail (cdr time))
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111 (lo (cons (car time) (if (numberp tail) tail (car tail))))
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112 probe)
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113 (while
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114 ;; Set PROBE to halfway between LO and HI, rounding down.
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115 ;; If PROBE equals LO, we are done.
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116 (let* ((lsum (+ (cdr lo) (cdr hi)))
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117 (hsum (+ (car lo) (car hi) (/ lsum base)))
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118 (hsumodd (logand 1 hsum)))
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119 (setq probe (cons (/ (- hsum hsumodd) 2)
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120 (/ (+ (* hsumodd base) (% lsum base)) 2)))
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121 (not (equal lo probe)))
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122 ;; Set either LO or HI to PROBE, depending on probe results.
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123 (if (eq (car (current-time-zone probe)) hi-utc-diff)
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124 (setq hi probe)
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125 (setq lo probe)))
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126 hi))))
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127
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128 (defun calendar-time-zone-daylight-rules (abs-date utc-diff)
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129 "Return daylight transition rule for ABS-DATE, UTC-DIFF sec offset from UTC.
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130 ABS-DIFF must specify a day that contains a daylight savings transition.
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131 The result has the proper form for calendar-daylight-savings-starts'."
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132 (let* ((date (calendar-gregorian-from-absolute abs-date))
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133 (weekday (% abs-date 7))
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134 (m (extract-calendar-month date))
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135 (d (extract-calendar-day date))
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136 (y (extract-calendar-year date))
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137 (last (calendar-last-day-of-month m y))
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138 (candidate-rules
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139 (append
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140 ;; Day D of month M.
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141 (list (list 'list m d 'year))
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142 ;; The first WEEKDAY of month M.
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143 (if (< d 8)
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144 (list (list 'calendar-nth-named-day 1 weekday m 'year)))
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145 ;; The last WEEKDAY of month M.
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146 (if (> d (- last 7))
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147 (list (list 'calendar-nth-named-day -1 weekday m 'year)))
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148 ;; The first WEEKDAY after day J of month M, for D-6 < J <= D.
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149 (let (l)
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150 (calendar-for-loop j from (max 2 (- d 6)) to (min d (- last 8)) do
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151 (setq l
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152 (cons
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153 (list 'calendar-nth-named-day 1 weekday m 'year j)
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154 l)))
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155 l)))
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156 (prevday-sec (- -1 utc-diff)) ;; last sec of previous local day
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157 (year (1+ y)))
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158 ;; Scan through the next few years until only one rule remains.
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159 (while
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160 (let ((rules candidate-rules)
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161 new-rules)
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162 (while
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163 (let*
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164 ((rule (car rules))
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165 (date
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166 ;; The following is much faster than
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167 ;; (calendar-absolute-from-gregorian (eval rule)).
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168 (cond ((eq (car rule) 'calendar-nth-named-day)
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169 (eval (cons 'calendar-nth-named-absday (cdr rule))))
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170 ((eq (car rule) 'calendar-gregorian-from-absolute)
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171 (eval (car (cdr rule))))
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172 (t (let ((g (eval rule)))
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173 (calendar-absolute-from-gregorian g))))))
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174 (or (equal
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175 (current-time-zone
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176 (calendar-time-from-absolute date prevday-sec))
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177 (current-time-zone
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178 (calendar-time-from-absolute (1+ date) prevday-sec)))
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179 (setq new-rules (cons rule new-rules)))
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180 (setq rules (cdr rules))))
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181 ;; If no rules remain, just use the first candidate rule;
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182 ;; it's wrong in general, but it's right for at least one year.
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183 (setq candidate-rules (if new-rules (nreverse new-rules)
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184 (list (car candidate-rules))))
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185 (setq year (1+ year))
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186 (cdr candidate-rules)))
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187 (car candidate-rules)))
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188
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189 (defun calendar-current-time-zone ()
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190 "Return UTC difference, dst offset, names and rules for current time zone.
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191
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192 Returns (UTC-DIFF DST-OFFSET STD-ZONE DST-ZONE DST-STARTS DST-ENDS
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193 DST-STARTS-TIME DST-ENDS-TIME), based on a heuristic probing of what the
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194 system knows:
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195
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196 UTC-DIFF is an integer specifying the number of minutes difference between
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197 standard time in the current time zone and Coordinated Universal Time
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198 (Greenwich Mean Time). A negative value means west of Greenwich.
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199 DST-OFFSET is an integer giving the daylight savings time offset in minutes.
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200 STD-ZONE is a string giving the name of the time zone when no seasonal time
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201 adjustment is in effect.
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202 DST-ZONE is a string giving the name of the time zone when there is a seasonal
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203 time adjustment in effect.
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204 DST-STARTS and DST-ENDS are sexps in the variable `year' giving the daylight
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205 savings time start and end rules, in the form expected by
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206 `calendar-daylight-savings-starts'.
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207 DST-STARTS-TIME and DST-ENDS-TIME are integers giving the number of minutes
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208 after midnight that daylight savings time starts and ends.
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209
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210 If the local area does not use a seasonal time adjustment, STD-ZONE and
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211 DST-ZONE are equal, and all the DST-* integer variables are 0.
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212
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213 Some operating systems cannot provide all this information to Emacs; in this
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214 case, `calendar-current-time-zone' returns a list containing nil for the data
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215 it can't find."
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216 (or
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217 calendar-current-time-zone-cache
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218 (setq
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219 calendar-current-time-zone-cache
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220 (let* ((t0 (current-time))
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221 (t0-zone (current-time-zone t0))
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222 (t0-utc-diff (car t0-zone))
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223 (t0-name (car (cdr t0-zone))))
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224 (if (not t0-utc-diff)
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225 ;; Little or no time zone information is available.
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226 (list nil nil t0-name t0-name nil nil nil nil)
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227 (let* ((t1 (calendar-next-time-zone-transition t0))
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228 (t2 (and t1 (calendar-next-time-zone-transition t1))))
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229 (if (not t2)
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230 ;; This locale does not have daylight savings time.
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231 (list (/ t0-utc-diff 60) 0 t0-name t0-name nil nil 0 0)
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232 ;; Use heuristics to find daylight savings parameters.
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233 (let* ((t1-zone (current-time-zone t1))
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234 (t1-utc-diff (car t1-zone))
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235 (t1-name (car (cdr t1-zone)))
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236 (t1-date-sec (calendar-absolute-from-time t1 t0-utc-diff))
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237 (t2-date-sec (calendar-absolute-from-time t2 t1-utc-diff))
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238 (t1-rules (calendar-time-zone-daylight-rules
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239 (car t1-date-sec) t0-utc-diff))
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240 (t2-rules (calendar-time-zone-daylight-rules
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241 (car t2-date-sec) t1-utc-diff))
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242 (t1-time (/ (cdr t1-date-sec) 60))
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243 (t2-time (/ (cdr t2-date-sec) 60)))
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244 (cons
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245 (/ (min t0-utc-diff t1-utc-diff) 60)
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246 (cons
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247 (/ (abs (- t0-utc-diff t1-utc-diff)) 60)
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248 (if (< t0-utc-diff t1-utc-diff)
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249 (list t0-name t1-name t1-rules t2-rules t1-time t2-time)
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250 (list t1-name t0-name t2-rules t1-rules t2-time t1-time)
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251 )))))))))))
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252
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253 ;;; The following six defvars relating to daylight savings time should NOT be
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254 ;;; marked to go into loaddefs.el where they would be evaluated when Emacs is
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255 ;;; dumped. These variables' appropriate values depend on the conditions under
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256 ;;; which the code is INVOKED; so it's inappropriate to initialize them when
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257 ;;; Emacs is dumped---they should be initialized when calendar.el is loaded.
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258 ;;; They default to US Eastern time if time zone info is not available.
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259
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260 (calendar-current-time-zone)
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261
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262 (defvar calendar-time-zone (or (car calendar-current-time-zone-cache) -300)
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263 "*Number of minutes difference between local standard time at
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264 `calendar-location-name' and Coordinated Universal (Greenwich) Time. For
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265 example, -300 for New York City, -480 for Los Angeles.")
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266
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267 (defvar calendar-daylight-time-offset
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268 (or (car (cdr calendar-current-time-zone-cache)) 60)
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269 "*Number of minutes difference between daylight savings and standard time.
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270
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271 If the locale never uses daylight savings time, set this to 0.")
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272
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273 (defvar calendar-standard-time-zone-name
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274 (or (car (nthcdr 2 calendar-current-time-zone-cache)) "EST")
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275 "*Abbreviated name of standard time zone at `calendar-location-name'.
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276 For example, \"EST\" in New York City, \"PST\" for Los Angeles.")
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277
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278 (defvar calendar-daylight-time-zone-name
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279 (or (car (nthcdr 3 calendar-current-time-zone-cache)) "EDT")
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280 "*Abbreviated name of daylight-savings time zone at `calendar-location-name'.
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281 For example, \"EDT\" in New York City, \"PDT\" for Los Angeles.")
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282
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283 ;;;###autoload
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284 (put 'calendar-daylight-savings-starts 'risky-local-variable t)
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285 (defvar calendar-daylight-savings-starts
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286 (or (car (nthcdr 4 calendar-current-time-zone-cache))
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287 (and (not (zerop calendar-daylight-time-offset))
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288 '(calendar-nth-named-day 1 0 4 year)))
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289 "*Sexp giving the date on which daylight savings time starts.
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290 This is an expression in the variable `year' whose value gives the Gregorian
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291 date in the form (month day year) on which daylight savings time starts. It is
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292 used to determine the starting date of daylight savings time for the holiday
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293 list and for correcting times of day in the solar and lunar calculations.
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294
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295 For example, if daylight savings time is mandated to start on October 1,
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296 you would set `calendar-daylight-savings-starts' to
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297
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298 '(10 1 year)
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299
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300 If it starts on the first Sunday in April, you would set it to
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301
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302 '(calendar-nth-named-day 1 0 4 year)
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303
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304 If the locale never uses daylight savings time, set this to nil.")
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305
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306 ;;;###autoload
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307 (put 'calendar-daylight-savings-ends 'risky-local-variable t)
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308 (defvar calendar-daylight-savings-ends
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309 (or (car (nthcdr 5 calendar-current-time-zone-cache))
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310 (and (not (zerop calendar-daylight-time-offset))
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311 '(calendar-nth-named-day -1 0 10 year)))
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312 "*Sexp giving the date on which daylight savings time ends.
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313 This is an expression in the variable `year' whose value gives the Gregorian
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314 date in the form (month day year) on which daylight savings time ends. It is
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315 used to determine the starting date of daylight savings time for the holiday
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316 list and for correcting times of day in the solar and lunar calculations.
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317
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318 For example, if daylight savings time ends on the last Sunday in October:
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319
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320 '(calendar-nth-named-day -1 0 10 year)
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321
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322 If the locale never uses daylight savings time, set this to nil.")
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323
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324 (defvar calendar-daylight-savings-starts-time
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325 (or (car (nthcdr 6 calendar-current-time-zone-cache)) 120)
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326 "*Number of minutes after midnight that daylight savings time starts.")
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327
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328 (defvar calendar-daylight-savings-ends-time
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329 (or (car (nthcdr 7 calendar-current-time-zone-cache))
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330 calendar-daylight-savings-starts-time)
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331 "*Number of minutes after midnight that daylight savings time ends.")
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332
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333 (provide 'cal-dst)
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334
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335 ;;; cal-dst.el ends here
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