Commit 723da6aa authored by Jay Belanger's avatar Jay Belanger

* doc/misc/calc.texi (Date Forms): Mention the customizable

Gregorian-Julian switch.
(Customizing Calc): Mention the variable `calc-gregorian-switch'.

* calc/calc.el (calc-gregorian-switch): Refresh the Calc buffer
after the variable is changed.
parent b6096e85
2012-11-22 Jay Belanger <jay.p.belanger@gmail.com>
* doc/misc/calc.texi (Date Forms): Mention the customizable
Gregorian-Julian switch.
(Customizing Calc): Mention the variable `calc-gregorian-switch'.
2012-11-17 Paul Eggert <eggert@cs.ucla.edu>
Calc now uses the Gregorian calendar for all dates,
......
......@@ -10987,10 +10987,10 @@ Input is flexible; date forms can be entered in any of the usual
notations for dates and times. @xref{Date Formats}.
Date forms are stored internally as numbers, specifically the number
of days since midnight on the morning of January 1 of the year 1 AD.
of days since midnight on the morning of December 31 of the year 1 BC.
If the internal number is an integer, the form represents a date only;
if the internal number is a fraction or float, the form represents
a date and time. For example, @samp{<6:00am Wed Jan 9, 1991>}
a date and time. For example, @samp{<6:00am Thu Jan 10, 1991>}
is represented by the number 726842.25. The standard precision of
12 decimal digits is enough to ensure that a (reasonable) date and
time can be stored without roundoff error.
......@@ -11014,42 +11014,55 @@ 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.
Gregorian calendar was invented (although that can be configured; see
below). 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 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, 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, 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.
(If it did, it would have to report a non-integer number of days
between, say, @samp{<12:00am Mon Jan 1, 1900>} and
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 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. For example, Great Britain and the
British colonies switched to the Gregorian calendar in September
1752, when the Julian calendar was eleven days behind the
Gregorian calendar. That year in Britain, the day after September 2
was September 14. To take another example, Russia did not adopt the
Gregorian calendar until 1918, and that year in Russia the day after
January 31 was February 14. Calc's reckoning therefore matches English
practice starting in 1752 and Russian practice starting in 1918, but
disagrees with earlier dates in both countries.
When the Julian calendar was introduced, it had January 1 as the first
day of the year. By the Middle Ages, many European countries
had changed the beginning of a new year to a different date, often to
a religious festival. Almost all countries reverted to using January 1
as the beginning of the year by the time they adopted the Gregorian
calendar.
Some calendars attempt to mimic the historical situation by using the
Gregorian calendar for recent dates and the Julian calendar for older
dates. The @code{cal} program in most Unix implementations does this,
for example. While January 1 wasn't always the beginning of a calendar
year, these hybrid calendars still use January 1 as the beginning of
the year even for older dates. The customizable variable
@code{calc-gregorian-switch} (@pxref{Customizing Calc}) can be set to
have Calc's date forms switch from the Julian to Gregorian calendar at
any specified date.
Today's timekeepers introduce an occasional ``leap second''.
These do not occur regularly and Calc does not take these minor
effects into account. (If it did, it would have to report a
non-integer number of days between, say,
@samp{<12:00am Mon Jan 1, 1900>} and
@samp{<12:00am Sat Jan 1, 2000>}.)
@cindex Julian day counting
......@@ -35590,6 +35603,20 @@ number of undo steps that will be preserved; if
be preserved. The default value of @code{calc-undo-length} is @expr{100}.
@end defvar
@defvar calc-gregorian-switch
See @ref{Date Forms}.@*
The variable @code{calc-gregorian-switch} is either a list of integers
@code{(@var{YEAR} @var{MONTH} @var{DAY})} or @code{nil}.
If it is @code{nil}, then Calc's date forms always represent Gregorian dates.
Otherwise, @code{calc-gregorian-switch} represents the date that the
calendar switches from Julian dates to Gregorian dates;
@code{(@var{YEAR} @var{MONTH} @var{DAY})} will be the first Gregorian
date. The customization buffer will offer several standard dates to
choose from, or the user can enter their own date.
The default value of @code{calc-gregorian-switch} is @code{nil}.
@end defvar
@node Reporting Bugs, Summary, Customizing Calc, Top
@appendix Reporting Bugs
2012-11-22 Jay Belanger <jay.p.belanger@gmail.com>
* calc/calc.el (calc-gregorian-switch): Refresh the Calc buffer
after the variable is changed.
2012-11-21 Daniel Colascione <dancol@dancol.org>
* progmodes/sql.el (sql-mode-font-lock-object-name): Support IF NOT EXISTS
......
......@@ -70,11 +70,18 @@ opinions.
Summary of changes to "Calc"
------- -- ------- -- ----
Emacs 24.4
* The date forms use the Gregorian calendar for all dates.
(Previously they were a combination of Julian and Gregorian
dates.) This can be configured with the customizable variable
`calc-gregorian-switch'.
Emacs 24.3
Algebraic simplification mode is now the default.
To restrict to the limited simplifications given by the former
default simplification mode, use `m I'.
* Algebraic simplification mode is now the default.
To restrict to the limited simplifications given by the former
default simplification mode, use `m I'.
Emacs 24.1
......
......@@ -508,7 +508,8 @@ the United States."
(integer :tag "Day")))
:set (lambda (symbol value)
(set-default symbol value)
(setq math-format-date-cache nil)))
(setq math-format-date-cache nil)
(calc-refresh)))
(defface calc-nonselected-face
'((t :inherit shadow
......
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