Commit 310e60d9 authored by Paul Eggert's avatar Paul Eggert Committed by Jay Belanger

Calc now uses the Gregorian calendar for all dates,

and uses January 1, 1 AD as its day number 1.
* doc/misc/calc.texi (Date Forms): Document this.

* lisp/calc/calc-forms.el (math-julian-date-beginning)
(math-julian-date-beginning-int): Implement this.
parent d7fe6352
2012-11-17 Paul Eggert <eggert@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.
2012-11-16 Glenn Morris <rgm@gnu.org>
* cl.texi (Function Bindings): Clarify that cl-flet is lexical.
......
......@@ -11010,35 +11010,41 @@ You can use the @kbd{v p} (@code{calc-pack}) and @kbd{v u}
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 not 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 @@ well, but Calc does not take these minor effects into account.
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
2012-11-17 Paul Eggert <eggert@cs.ucla.edu>
* calc/calc-forms.el (math-julian-date-beginning)
(math-julian-date-beginning-int): Implement [new date numbering].
2012-11-17 Juanma Barranquero <lekktu@gmail.com>
* descr-text.el (quail-find-key):
......
......@@ -656,13 +656,13 @@ in the Gregorian calendar."
(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 '(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 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-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)
......
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