GNU bug report logs - #12633
Make Calc more compatible with Calendar

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: bug-gnu-emacs <at> gnu.org
Subject: Make Calc more compatible with Calendar
Date: Fri, 12 Oct 2012 22:57:23 -0700

Tags: patch

Here's a patch to implement the proposal discussed recently
on emacs-devel, to make Calc more compatible with Calendar
with respect to the Gregorian and Julian calendars.  This
is relative to trunk bzr 110529.

=== modified file 'doc/misc/ChangeLog'
--- doc/misc/ChangeLog	2012-10-09 12:51:21 +0000
+++ doc/misc/ChangeLog	2012-10-13 05:39:41 +0000
@@ -1,3 +1,9 @@
+2012-10-13  Paul Eggert  <eggert <at> cs.ucla.edu>
+
+	Calc now uses the Gregorian calendar for all dates,
+	and uses January 1, 1 AD as its day number 1.
+	* calc.texi (Date Forms): Document this.
+
 2012-10-09  Michael Albinus  <michael.albinus <at> gmx.de>
 
 	* trampver.texi: Update release number.

=== modified file 'doc/misc/calc.texi'
--- doc/misc/calc.texi	2012-10-05 07:38:05 +0000
+++ doc/misc/calc.texi	2012-10-13 05:13:39 +0000
@@ -11010,35 +11010,41 @@
 of a date form.  @xref{Packing and Unpacking}.
 
 Date forms can go arbitrarily far into the future or past.  Negative
-year numbers represent years BC.  Calc uses a combination of the
-Gregorian and Julian calendars, following the history of Great
-Britain and the British colonies.  This is the same calendar that
-is used by the @code{cal} program in most Unix implementations.
+year numbers represent years BC.  There is no ``year 0''; the day
+before @samp{<Mon Jan 1, +1>} is @samp{<Sun Dec 31, -1>}.  These are
+days 1 and 0 respectively in Calc's internal numbering scheme.  The
+Gregorian calendar is used for all dates, including dates before the
+Gregorian calendar was invented.  Thus Calc's use of the day number
+@mathit{-10000} to represent August 15, 28 BC should be taken with a
+grain of salt.
 
 @cindex Julian calendar
 @cindex Gregorian calendar
 Some historical background:  The Julian calendar was created by
-Julius Caesar in the year 46 BC as an attempt to fix the gradual
-drift caused by the lack of leap years in the calendar used
-until that time.  The Julian calendar introduced an extra day in
+Julius Caesar in the year 46 BC as an attempt to fix the confusion
+caused by the irregular Roman calendar that was used before that time.
+The Julian calendar introduced an extra day in
 all years divisible by four.  After some initial confusion, the
-calendar was adopted around the year we call 8 AD.  Some centuries
+calendar was adopted around the year we call 8 AD, although the years were
+numbered differently and did necessarily begin on January 1.  Some centuries
 later it became apparent that the Julian year of 365.25 days was
 itself not quite right.  In 1582 Pope Gregory XIII introduced the
 Gregorian calendar, which added the new rule that years divisible
 by 100, but not by 400, were not to be considered leap years
 despite being divisible by four.  Many countries delayed adoption
-of the Gregorian calendar because of religious differences;
-in Britain it was put off until the year 1752, by which time
-the Julian calendar had fallen eleven days behind the true
-seasons.  So the switch to the Gregorian calendar in early
-September 1752 introduced a discontinuity:  The day after
-Sep 2, 1752 is Sep 14, 1752.  Calc follows this convention.
-To take another example, Russia waited until 1918 before
-adopting the new calendar, and thus needed to remove thirteen
-days (between Feb 1, 1918 and Feb 14, 1918).  This means that
-Calc's reckoning will be inconsistent with Russian history between
-1752 and 1918, and similarly for various other countries.
+of the Gregorian calendar because of religious differences, and
+used differing year numbers and start-of-year for other reasons;
+for example, in early 1752 England changed the start of its year from
+March 25 to January 1, and in September it switched to the Gregorian
+calendar: in England, the day after December 31, 1750 was January 1,
+1750 and the day after March 24, 1750 was March 25, 1751, but the day
+after December 31, 1751 was January 1, 1752 and the day after
+September 2, 1752 was September 14, 1752.  To take another example,
+Russia switched both year numbering and start-of-year in 1700, but did
+not adopt the Gregorian calendar until 1918.  Calc's reckoning
+therefore matches English practice starting in 1752 and Russian
+practice starting in 1918, but disagrees with earlier dates in both
+countries.
 
 Today's timekeepers introduce an occasional ``leap second'' as
 well, but Calc does not take these minor effects into account.
@@ -11046,15 +11052,6 @@
 between, say, @samp{<12:00am Mon Jan 1, 1900>} and
 @samp{<12:00am Sat Jan 1, 2000>}.)
 
-Calc uses the Julian calendar for all dates before the year 1752,
-including dates BC when the Julian calendar technically had not
-yet been invented.  Thus the claim that day number @mathit{-10000} is
-called ``August 16, 28 BC'' should be taken with a grain of salt.
-
-Please note that there is no ``year 0''; the day before
-@samp{<Sat Jan 1, +1>} is @samp{<Fri Dec 31, -1>}.  These are
-days 0 and @mathit{-1} respectively in Calc's internal numbering scheme.
-
 @cindex Julian day counting
 Another day counting system in common use is, confusingly, also called
 ``Julian.''  The Julian day number is the numbers of days since

=== modified file 'etc/ChangeLog'
--- etc/ChangeLog	2012-10-11 11:26:26 +0000
+++ etc/ChangeLog	2012-10-13 05:40:15 +0000
@@ -1,3 +1,9 @@
+2012-10-13  Paul Eggert  <eggert <at> cs.ucla.edu>
+
+	Calc now uses the Gregorian calendar for all dates,
+	and uses January 1, 1 AD as its day number 1.
+	* NEWS (Calc): Document this.
+
 2012-10-11  Kenichi Handa  <handa <at> gnu.org>
 
 	* charsets/CNS-2.map, charsets/CNS-3.map, charsets/CNS-4.map,

=== modified file 'etc/NEWS'
--- etc/NEWS	2012-10-13 01:18:52 +0000
+++ etc/NEWS	2012-10-13 05:24:37 +0000
@@ -286,6 +286,14 @@
 *** Option `Buffer-menu-buffer+size-width' is now obsolete.
 Use `Buffer-menu-name-width' and `Buffer-menu-size-width' instead.
 
+** Calc
+
+*** Calc now uses the Gregorian calendar for all dates, and uses
+January 1, 1 AD as its day number 1.  Previously Calc used the Julian
+calendar for dates before September 14, 1752, and it used December 31,
+1 BC as its day number 1; the new scheme is more consistent with
+Calendar's calendrical system and day numbering.
+
 ** Calendar
 
 +++

=== modified file 'lisp/ChangeLog'
--- lisp/ChangeLog	2012-10-12 20:11:50 +0000
+++ lisp/ChangeLog	2012-10-13 05:26:27 +0000
@@ -1,3 +1,18 @@
+2012-10-13  Paul Eggert  <eggert <at> cs.ucla.edu>
+
+	Calc now uses the Gregorian calendar for all dates,
+	and uses January 1, 1 AD as its day number 1.  Previously Calc
+	used the Julian calendar for dates before September 14, 1752, and
+	it used December 31, 1 BC as its day number 1; the new scheme is
+	more consistent with Calendar's calendrical system and day numbering.
+	Problem reported by Jay Belanger in
+	<http://lists.gnu.org/archive/html/emacs-devel/2012-10/msg00298.html>.
+	* calc/calc-forms.el (math-date-to-dt, math-leap-year-p)
+	(math-day-number, math-absolute-from-date)
+	(math-julian-date-beginning, math-julian-date-beginning-int)
+	(calcFunc-newmonth, calcFunc-newyear):
+	Implement this.
+
 2012-10-12  Stefan Monnier  <monnier <at> iro.umontreal.ca>
 
 	* help-fns.el (describe-variable, describe-function-1):

=== modified file 'lisp/calc/calc-forms.el'
--- lisp/calc/calc-forms.el	2012-09-17 05:41:04 +0000
+++ lisp/calc/calc-forms.el	2012-10-13 05:33:53 +0000
@@ -369,12 +369,11 @@
 
 ;;; Some of these functions are adapted from Edward Reingold's "calendar.el".
 ;;; These versions are rewritten to use arbitrary-size integers.
-;;; The Julian calendar is used up to 9/2/1752, after which the Gregorian
-;;; calendar is used; the first day after 9/2/1752 is 9/14/1752.
+;;; The Gregorian calendar is used even for older dates.
 
 ;;; A numerical date is the number of days since midnight on
-;;; the morning of January 1, 1 A.D.  If the date is a non-integer,
-;;; it represents a specific date and time.
+;;; the morning of December 31, 1 BC (Gregorian).
+;;; If the date is a non-integer, it represents a specific date and time.
 ;;; A "dt" is a list of the form, (year month day), corresponding to
 ;;; an integer code, or (year month day hour minute second), corresponding
 ;;; to a non-integer code.
@@ -389,9 +388,9 @@
 	 (time (nth 1 parts))
 	 (month 1)
 	 day
-	 (year (math-quotient (math-add date (if (Math-lessp date 711859)
+	 (year (math-quotient (math-add date (if (Math-lessp date 711858)
 						 365  ; for speed, we take
-					       -108)) ; >1950 as a special case
+					       -108)) ; >=1950 as a special case
 			      (if (math-negp value) 366 365)))
 					; this result may be an overestimate
 	 temp)
@@ -399,8 +398,6 @@
       (setq year (math-add year -1)))
     (if (eq year 0) (setq year -1))
     (setq date (1+ (math-sub date temp)))
-    (and (eq year 1752) (>= date 247)
-	 (setq date (+ date 11)))
     (setq temp (if (math-leap-year-p year)
 		   [1 32 61 92 122 153 183 214 245 275 306 336 999]
 		 [1 32 60 91 121 152 182 213 244 274 305 335 999]))
@@ -447,13 +444,11 @@
   (nth 5 (decode-time)))
 
 (defun math-leap-year-p (year)
-  (if (Math-lessp year 1752)
       (if (math-negp year)
-	  (= (math-imod (math-neg year) 4) 1)
-	(= (math-imod year 4) 0))
+      (setq year (math-add 1 year)))
     (setq year (math-imod year 400))
     (or (and (= (% year 4) 0) (/= (% year 100) 0))
-	(= year 0))))
+      (= year 0)))
 
 (defun math-days-in-month (year month)
   (if (and (= month 2) (math-leap-year-p year))
@@ -467,10 +462,6 @@
 	  (setq day-of-year (- day-of-year (/ (+ 23 (* 4 month)) 10)))
 	  (if (math-leap-year-p year)
 	      (setq day-of-year (1+ day-of-year)))))
-    (and (eq year 1752)
-	 (or (> month 9)
-	     (and (= month 9) (>= day 14)))
-	 (setq day-of-year (- day-of-year 11)))
     day-of-year))
 
 (defun math-absolute-from-date (year month day)
@@ -483,15 +474,12 @@
 				    (math-sub 365
 					      (math-quotient (math-sub 3 year)
 							     4)))))
-	      (if (or (Math-lessp year 1753)
-		      (and (eq year 1752) (<= month 9)))
-		  1
 		(let ((correction (math-mul (math-quotient yearm1 100) 3)))
 		  (let ((res (math-idivmod correction 4)))
-		    (math-add (if (= (cdr res) 0)
-				  -1
-				0)
-			      (car res))))))))
+		  (math-add (if (zerop (cdr res))
+				0
+			      1)
+			    (car res)))))))
 
 
 ;;; It is safe to redefine these in your init file to use a different
@@ -548,13 +536,13 @@
 	       (setcdr math-fd-dt nil))
 	  fmt))))
 
-(defconst math-julian-date-beginning '(float 17214235 -1)
-  "The beginning of the Julian calendar,
-as measured in the number of days before January 1 of the year 1AD.")
-
-(defconst math-julian-date-beginning-int 1721424
-  "The beginning of the Julian calendar,
-as measured in the integer number of days before January 1 of the year 1AD.")
+(defconst math-julian-date-beginning '(float 17214225 -1)
+  "The beginning of the Julian date calendar,
+as measured in the number of days before December 31, 1 BC (Gregorian).")
+
+(defconst math-julian-date-beginning-int 1721423
+  "The beginning of the Julian date calendar,
+as measured in the integer number of days before December 31, 1 BC (Gregorian).")
 
 (defun math-format-date-part (x)
   (cond ((stringp x)
@@ -1437,8 +1425,6 @@
   (let ((dt (math-date-to-dt date)))
     (if (or (= day 0) (> day (math-days-in-month (car dt) (nth 1 dt))))
 	(setq day (math-days-in-month (car dt) (nth 1 dt))))
-    (and (eq (car dt) 1752) (= (nth 1 dt) 9)
-	 (if (>= day 14) (setq day (- day 11))))
     (list 'date (math-add (math-dt-to-date (list (car dt) (nth 1 dt) 1))
 			  (1- day)))))
 
@@ -1448,8 +1434,7 @@
   (or (integerp day) (math-reject-arg day 'fixnump))
   (let ((dt (math-date-to-dt date)))
     (if (and (>= day 0) (<= day 366))
-	(let ((max (if (eq (car dt) 1752) 355
-		     (if (math-leap-year-p (car dt)) 366 365))))
+	(let ((max (if (math-leap-year-p (car dt)) 366 365)))
 	  (if (or (= day 0) (> day max)) (setq day max))
 	  (list 'date (math-add (math-dt-to-date (list (car dt) 1 1))
 				(1- day))))
@@ -1523,7 +1508,7 @@
 (defun calcFunc-holiday (a)
   (if (cdr (math-to-business-day a)) 1 0))
 
-;;; Compute the number of business days since Jan 1, 1 AD.
+;;; Compute the number of business days since December 31, 1 BC (Gregorian).
 
 (defun math-to-business-day (date &optional need-year)
   (if (eq (car-safe date) 'date)
@@ -1564,7 +1549,8 @@
     (cons (math-add (math-sub day delta) time) holiday)))
 
 
-;;; Compute the date a certain number of business days since Jan 1, 1 AD.
+;;; Compute the date from a certain number of business days since
+;;; December 31, 1 BC (Gregorian).
 ;;; If this returns nil, holiday table was adjusted; redo calculation.
 
 (defun math-from-business-day (num)

Message #10 received at 12633-done <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: 12633-done <at> debbugs.gnu.org
Subject: patch installed
Date: Thu, 22 Nov 2012 23:33:59 -0800

This patch has been installed into the trunk (bzr 110930)
so I'm marking it as done.

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