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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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16
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23 ;; along with XEmacs; see the file COPYING. If not, write to the
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24 ;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
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25 ;; Boston, MA 02111-1307, USA.
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26
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27 ;;; Commentary:
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28
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29 ;; This collection of functions implements the features of calendar.el and
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30 ;; holiday.el that deal with daylight savings time.
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31
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32 ;; Comments, corrections, and improvements should be sent to
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33 ;; Edward M. Reingold Department of Computer Science
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34 ;; (217) 333-6733 University of Illinois at Urbana-Champaign
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35 ;; reingold@cs.uiuc.edu 1304 West Springfield Avenue
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36 ;; Urbana, Illinois 61801
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37
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38 ;;; Code:
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39
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40 (require 'calendar)
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41
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42 (defvar calendar-current-time-zone-cache nil
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43 "Cache for result of calendar-current-time-zone.")
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44
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45 (defvar calendar-system-time-basis
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46 (calendar-absolute-from-gregorian '(1 1 1970))
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47 "Absolute date of starting date of system clock.")
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48
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49 (defun calendar-absolute-from-time (x utc-diff)
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50 "Absolute local date of time X; local time is UTC-DIFF seconds from UTC.
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51
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52 X is (HIGH . LOW) or (HIGH LOW . IGNORED) where HIGH and LOW are the
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53 high and low 16 bits, respectively, of the number of seconds since
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54 1970-01-01 00:00:00 UTC, ignoring leap seconds.
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55
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56 Returns the pair (ABS-DATE . SECONDS) where SECONDS after local midnight on
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57 absolute date ABS-DATE is the equivalent moment to X."
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58 (let* ((h (car x))
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59 (xtail (cdr x))
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60 (l (+ utc-diff (if (numberp xtail) xtail (car xtail))))
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61 (u (+ (* 512 (mod h 675)) (floor l 128))))
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62 ;; Overflow is a terrible thing!
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63 (cons (+ calendar-system-time-basis
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64 ;; floor((2^16 h +l) / (60*60*24))
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65 (* 512 (floor h 675)) (floor u 675))
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66 ;; (2^16 h +l) mod (60*60*24)
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67 (+ (* (mod u 675) 128) (mod l 128)))))
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68
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69 (defun calendar-time-from-absolute (abs-date s)
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70 "Time of absolute date ABS-DATE, S seconds after midnight.
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71
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72 Returns the pair (HIGH . LOW) where HIGH and LOW are the high and low
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73 16 bits, respectively, of the number of seconds 1970-01-01 00:00:00 UTC,
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74 ignoring leap seconds, that is the equivalent moment to S seconds after
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75 midnight UTC on absolute date ABS-DATE."
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76 (let* ((a (- abs-date calendar-system-time-basis))
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77 (u (+ (* 163 (mod a 512)) (floor s 128))))
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78 ;; Overflow is a terrible thing!
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79 (cons
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80 ;; floor((60*60*24*a + s) / 2^16)
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81 (+ a (* 163 (floor a 512)) (floor u 512))
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82 ;; (60*60*24*a + s) mod 2^16
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83 (+ (* 128 (mod u 512)) (mod s 128)))))
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84
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85 (defun calendar-next-time-zone-transition (time)
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86 "Return the time of the next time zone transition after TIME.
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87 Both TIME and the result are acceptable arguments to current-time-zone.
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88 Return nil if no such transition can be found."
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89 (let* ((base 65536);; 2^16 = base of current-time output
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90 (quarter-multiple 120);; approx = (seconds per quarter year) / base
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91 (time-zone (current-time-zone time))
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92 (time-utc-diff (car time-zone))
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93 hi
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94 hi-zone
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95 (hi-utc-diff time-utc-diff)
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96 (quarters '(2 1 3)))
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97 ;; Heuristic: probe the time zone offset in the next three calendar
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98 ;; quarters, looking for a time zone offset different from TIME.
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99 (while (and quarters (eq time-utc-diff hi-utc-diff))
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100 (setq hi (cons (+ (car time) (* (car quarters) quarter-multiple)) 0))
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101 (setq hi-zone (current-time-zone hi))
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102 (setq hi-utc-diff (car hi-zone))
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103 (setq quarters (cdr quarters)))
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104 (and
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105 time-utc-diff
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106 hi-utc-diff
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107 (not (eq time-utc-diff hi-utc-diff))
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108 ;; Now HI is after the next time zone transition.
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109 ;; Set LO to TIME, and then binary search to increase LO and decrease HI
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110 ;; until LO is just before and HI is just after the time zone transition.
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111 (let* ((tail (cdr time))
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112 (lo (cons (car time) (if (numberp tail) tail (car tail))))
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113 probe)
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114 (while
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115 ;; Set PROBE to halfway between LO and HI, rounding down.
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116 ;; If PROBE equals LO, we are done.
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117 (let* ((lsum (+ (cdr lo) (cdr hi)))
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118 (hsum (+ (car lo) (car hi) (/ lsum base)))
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119 (hsumodd (logand 1 hsum)))
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120 (setq probe (cons (/ (- hsum hsumodd) 2)
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121 (/ (+ (* hsumodd base) (% lsum base)) 2)))
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122 (not (equal lo probe)))
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123 ;; Set either LO or HI to PROBE, depending on probe results.
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124 (if (eq (car (current-time-zone probe)) hi-utc-diff)
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125 (setq hi probe)
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126 (setq lo probe)))
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127 hi))))
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128
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129 (defun calendar-time-zone-daylight-rules (abs-date utc-diff)
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130 "Return daylight transition rule for ABS-DATE, UTC-DIFF sec offset from UTC.
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131 ABS-DIFF must specify a day that contains a daylight savings transition.
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132 The result has the proper form for calendar-daylight-savings-starts'."
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133 (let* ((date (calendar-gregorian-from-absolute abs-date))
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134 (weekday (% abs-date 7))
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135 (m (extract-calendar-month date))
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136 (d (extract-calendar-day date))
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137 (y (extract-calendar-year date))
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138 (last (calendar-last-day-of-month m y))
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139 (candidate-rules
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140 (append
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141 ;; Day D of month M.
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142 (list (list 'list m d 'year))
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143 ;; The first WEEKDAY of month M.
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144 (if (< d 8)
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145 (list (list 'calendar-nth-named-day 1 weekday m 'year)))
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146 ;; The last WEEKDAY of month M.
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147 (if (> d (- last 7))
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148 (list (list 'calendar-nth-named-day -1 weekday m 'year)))
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149 ;; The first WEEKDAY after day J of month M, for D-6 < J <= D.
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150 (let (l)
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151 (calendar-for-loop j from (max 2 (- d 6)) to (min d (- last 8)) do
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152 (setq l
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153 (cons
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154 (list 'calendar-nth-named-day 1 weekday m 'year j)
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155 l)))
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156 l)))
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157 (prevday-sec (- -1 utc-diff)) ;; last sec of previous local day
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158 (year (1+ y)))
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159 ;; Scan through the next few years until only one rule remains.
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160 (while
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161 (let ((rules candidate-rules)
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162 new-rules)
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163 (while
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164 (let*
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165 ((rule (car rules))
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166 (date
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167 ;; The following is much faster than
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168 ;; (calendar-absolute-from-gregorian (eval rule)).
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169 (cond ((eq (car rule) 'calendar-nth-named-day)
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170 (eval (cons 'calendar-nth-named-absday (cdr rule))))
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171 ((eq (car rule) 'calendar-gregorian-from-absolute)
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172 (eval (car (cdr rule))))
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173 (t (let ((g (eval rule)))
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174 (calendar-absolute-from-gregorian g))))))
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175 (or (equal
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176 (current-time-zone
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177 (calendar-time-from-absolute date prevday-sec))
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178 (current-time-zone
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179 (calendar-time-from-absolute (1+ date) prevday-sec)))
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180 (setq new-rules (cons rule new-rules)))
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181 (setq rules (cdr rules))))
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182 ;; If no rules remain, just use the first candidate rule;
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183 ;; it's wrong in general, but it's right for at least one year.
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184 (setq candidate-rules (if new-rules (nreverse new-rules)
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185 (list (car candidate-rules))))
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186 (setq year (1+ year))
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187 (cdr candidate-rules)))
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188 (car candidate-rules)))
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189
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190 (defun calendar-current-time-zone ()
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191 "Return UTC difference, dst offset, names and rules for current time zone.
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192
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193 Returns (UTC-DIFF DST-OFFSET STD-ZONE DST-ZONE DST-STARTS DST-ENDS
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194 DST-STARTS-TIME DST-ENDS-TIME), based on a heuristic probing of what the
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195 system knows:
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196
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197 UTC-DIFF is an integer specifying the number of minutes difference between
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198 standard time in the current time zone and Coordinated Universal Time
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199 (Greenwich Mean Time). A negative value means west of Greenwich.
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200 DST-OFFSET is an integer giving the daylight savings time offset in minutes.
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201 STD-ZONE is a string giving the name of the time zone when no seasonal time
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202 adjustment is in effect.
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203 DST-ZONE is a string giving the name of the time zone when there is a seasonal
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204 time adjustment in effect.
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205 DST-STARTS and DST-ENDS are sexps in the variable `year' giving the daylight
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206 savings time start and end rules, in the form expected by
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207 `calendar-daylight-savings-starts'.
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208 DST-STARTS-TIME and DST-ENDS-TIME are integers giving the number of minutes
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209 after midnight that daylight savings time starts and ends.
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210
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211 If the local area does not use a seasonal time adjustment, STD-ZONE and
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212 DST-ZONE are equal, and all the DST-* integer variables are 0.
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213
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214 Some operating systems cannot provide all this information to Emacs; in this
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215 case, `calendar-current-time-zone' returns a list containing nil for the data
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216 it can't find."
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217 (or
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218 calendar-current-time-zone-cache
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219 (setq
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220 calendar-current-time-zone-cache
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221 (let* ((t0 (current-time))
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222 (t0-zone (current-time-zone t0))
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223 (t0-utc-diff (car t0-zone))
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224 (t0-name (car (cdr t0-zone))))
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225 (if (not t0-utc-diff)
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226 ;; Little or no time zone information is available.
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227 (list nil nil t0-name t0-name nil nil nil nil)
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228 (let* ((t1 (calendar-next-time-zone-transition t0))
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229 (t2 (and t1 (calendar-next-time-zone-transition t1))))
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230 (if (not t2)
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231 ;; This locale does not have daylight savings time.
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232 (list (/ t0-utc-diff 60) 0 t0-name t0-name nil nil 0 0)
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233 ;; Use heuristics to find daylight savings parameters.
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234 (let* ((t1-zone (current-time-zone t1))
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235 (t1-utc-diff (car t1-zone))
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236 (t1-name (car (cdr t1-zone)))
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237 (t1-date-sec (calendar-absolute-from-time t1 t0-utc-diff))
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238 (t2-date-sec (calendar-absolute-from-time t2 t1-utc-diff))
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239 (t1-rules (calendar-time-zone-daylight-rules
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240 (car t1-date-sec) t0-utc-diff))
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241 (t2-rules (calendar-time-zone-daylight-rules
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242 (car t2-date-sec) t1-utc-diff))
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243 (t1-time (/ (cdr t1-date-sec) 60))
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244 (t2-time (/ (cdr t2-date-sec) 60)))
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245 (cons
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246 (/ (min t0-utc-diff t1-utc-diff) 60)
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247 (cons
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248 (/ (abs (- t0-utc-diff t1-utc-diff)) 60)
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249 (if (< t0-utc-diff t1-utc-diff)
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250 (list t0-name t1-name t1-rules t2-rules t1-time t2-time)
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251 (list t1-name t0-name t2-rules t1-rules t2-time t1-time)
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252 )))))))))))
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253
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254 ;;; The following six defvars relating to daylight savings time should NOT be
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255 ;;; marked to go into loaddefs.el where they would be evaluated when Emacs is
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256 ;;; dumped. These variables' appropriate values depend on the conditions under
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257 ;;; which the code is INVOKED; so it's inappropriate to initialize them when
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258 ;;; Emacs is dumped---they should be initialized when calendar.el is loaded.
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259 ;;; They default to US Eastern time if time zone info is not available.
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260
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261 (calendar-current-time-zone)
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262
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263 (defvar calendar-time-zone (or (car calendar-current-time-zone-cache) -300)
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264 "*Number of minutes difference between local standard time at
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265 `calendar-location-name' and Coordinated Universal (Greenwich) Time. For
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266 example, -300 for New York City, -480 for Los Angeles.")
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267
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268 (defvar calendar-daylight-time-offset
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269 (or (car (cdr calendar-current-time-zone-cache)) 60)
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270 "*Number of minutes difference between daylight savings and standard time.
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271
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272 If the locale never uses daylight savings time, set this to 0.")
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273
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274 (defvar calendar-standard-time-zone-name
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275 (or (car (nthcdr 2 calendar-current-time-zone-cache)) "EST")
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276 "*Abbreviated name of standard time zone at `calendar-location-name'.
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277 For example, \"EST\" in New York City, \"PST\" for Los Angeles.")
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278
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279 (defvar calendar-daylight-time-zone-name
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280 (or (car (nthcdr 3 calendar-current-time-zone-cache)) "EDT")
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281 "*Abbreviated name of daylight-savings time zone at `calendar-location-name'.
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282 For example, \"EDT\" in New York City, \"PDT\" for Los Angeles.")
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283
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284 ;;;###autoload
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285 (put 'calendar-daylight-savings-starts 'risky-local-variable t)
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286 (defvar calendar-daylight-savings-starts
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287 (or (car (nthcdr 4 calendar-current-time-zone-cache))
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288 (and (not (zerop calendar-daylight-time-offset))
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289 '(calendar-nth-named-day 1 0 4 year)))
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290 "*Sexp giving the date on which daylight savings time starts.
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291 This is an expression in the variable `year' whose value gives the Gregorian
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292 date in the form (month day year) on which daylight savings time starts. It is
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293 used to determine the starting date of daylight savings time for the holiday
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294 list and for correcting times of day in the solar and lunar calculations.
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295
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296 For example, if daylight savings time is mandated to start on October 1,
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297 you would set `calendar-daylight-savings-starts' to
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298
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299 '(10 1 year)
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300
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301 If it starts on the first Sunday in April, you would set it to
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302
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303 '(calendar-nth-named-day 1 0 4 year)
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304
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305 If the locale never uses daylight savings time, set this to nil.")
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306
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307 ;;;###autoload
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308 (put 'calendar-daylight-savings-ends 'risky-local-variable t)
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309 (defvar calendar-daylight-savings-ends
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310 (or (car (nthcdr 5 calendar-current-time-zone-cache))
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311 (and (not (zerop calendar-daylight-time-offset))
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312 '(calendar-nth-named-day -1 0 10 year)))
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313 "*Sexp giving the date on which daylight savings time ends.
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314 This is an expression in the variable `year' whose value gives the Gregorian
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315 date in the form (month day year) on which daylight savings time ends. It is
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316 used to determine the starting date of daylight savings time for the holiday
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317 list and for correcting times of day in the solar and lunar calculations.
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318
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319 For example, if daylight savings time ends on the last Sunday in October:
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320
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321 '(calendar-nth-named-day -1 0 10 year)
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322
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323 If the locale never uses daylight savings time, set this to nil.")
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324
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325 (defvar calendar-daylight-savings-starts-time
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326 (or (car (nthcdr 6 calendar-current-time-zone-cache)) 120)
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327 "*Number of minutes after midnight that daylight savings time starts.")
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328
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329 (defvar calendar-daylight-savings-ends-time
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330 (or (car (nthcdr 7 calendar-current-time-zone-cache))
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331 calendar-daylight-savings-starts-time)
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332 "*Number of minutes after midnight that daylight savings time ends.")
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333
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334 (provide 'cal-dst)
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335
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336 ;;; cal-dst.el ends here
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