3 files changed, 62 insertions, 52 deletions

diff --git a/lisp/calc/README b/lisp/calc/README
index 25d1a5e9b58..638b482a60a 100644
--- a/lisp/calc/README
+++ b/lisp/calc/README
@@ -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
 
diff --git a/lisp/calc/calc-forms.el b/lisp/calc/calc-forms.el
index 709250f9ba9..98b22550f75 100644
--- a/lisp/calc/calc-forms.el
+++ b/lisp/calc/calc-forms.el
@@ -371,9 +371,10 @@
 ;;; These versions are rewritten to use arbitrary-size integers.
 
 ;;; A numerical date is the number of days since midnight on
-;;; the morning of December 31, 1 B.C.  Emacs's calendar refers to such
-;;; a date as an absolute date, some function names also use that 
-;;; terminology.  If the date is a non-integer, it represents a specific date and time.
+;;; the morning of December 31, 1 B.C. (Gregorian) or January 2, 1 A.D. (Julian).
+;;; Emacs's calendar refers to such a date as an absolute date, some Calc function
+;;; names also use that terminology.  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.
@@ -408,8 +409,8 @@ DATE is the number of days since December 31, -1 in the Gregorian calendar."
   (let* ((month 1)
          day
          (year (math-quotient (math-add date (if (Math-lessp date 711859)
-                                                 365  ; for speed, we take
-                                               -108)) ; >1950 as a special case
+                                                 367  ; for speed, we take
+                                               -106)) ; >1950 as a special case
                               (if (math-negp date) 366 365)))
                                         ; this result may be an overestimate
          temp)
@@ -494,6 +495,8 @@ Gregorian calendar."
       (if (math-negp year)
           (= (math-imod (math-neg year) 4) 1)
         (= (math-imod year 4) 0))
+    (if (math-negp year)
+        (setq year (math-sub -1 year)))
     (setq year (math-imod year 400))
     (or (and (= (% year 4) 0) (/= (% year 100) 0))
         (= year 0))))
diff --git a/lisp/calc/calc.el b/lisp/calc/calc.el
index aeca45ebf26..58eabf9bcec 100644
--- a/lisp/calc/calc.el
+++ b/lisp/calc/calc.el
@@ -466,50 +466,6 @@ to be identified as that note."
 
 (defvar math-format-date-cache) ; calc-forms.el
 
-;; Dates that are built-in options for `calc-gregorian-switch' should be
-;; (YEAR MONTH DAY math-date-from-gregorian-dt(YEAR MONTH DAY)) for speed.
-(defcustom calc-gregorian-switch nil
-  "The first day the Gregorian calendar is used by Calc's date forms.
-This is `nil' (the default) if the Gregorian calendar is the only one used.
-Otherwise, it should be a list `(YEAR MONTH DAY)' when Calc begins to use
-the Gregorian calendar; Calc will use the Julian calendar for earlier dates.
-The dates in which different regions of the world began to use the
-Gregorian calendar vary quite a bit, even within a single country.
-If you want Calc's date forms to switch between the Julian and
-Gregorian calendar, you can specify the date or choose from several
-common choices.  Some of these choices should be taken with a grain
-of salt; for example different parts of France changed calendars at
-different times, and Sweden's change to the Gregorian calendar was
-complicated.  Also, the boundaries of the countries were different at
-the times of the calendar changes than they are now.
-The Vatican decided that the Gregorian calendar should take effect
-on 15 October 1582 (Gregorian), and many Catholic countries made
-the change then.  Great Britian and its colonies had the Gregorian
-calendar take effect on 14 September 1752 (Gregorian); this includes
-the United States."
-  :group 'calc
-  :version "24.4"
-  :type '(choice (const :tag "Always use the Gregorian calendar" nil)
-                 (const :tag "Great Britian and the US (1752 9 14)" (1752 9 14 639797))
-                 (const :tag "Vatican (1582 10 15)" (1582 10 15 577736))
-                 (const :tag "Czechoslovakia (1584 1 17)" (1584 1 17 578195))
-                 (const :tag "Denmark (1700 3 1)" (1700 3 1 620607))
-                 (const :tag "France (1582 12 20)" (1582 12 20 577802))
-                 (const :tag "Hungary (1587 11 1)" (1587 11 1 579579))
-                 (const :tag "Luxemburg (1582 12 25)" (1582 12 25 577807))
-                 (const :tag "Romania (1919 4 14)" (1919 4 14 700638))
-                 (const :tag "Russia (1918 2 14)" (1918 2 14 700214))
-                 (const :tag "Sweden (1753 3 1)" (1753 3 1 639965))
-                 (const :tag "Switzerland (Catholic) (1584 1 22)" (1584 1 22 578200))
-                 (const :tag "Switzerland (Protestant) (1701 1 12)" (1701 1 12 620924))
-                 (list :tag "(YEAR MONTH DAY)"
-                       (integer :tag "Year")
-                       (integer :tag "Month (integer)")
-                       (integer :tag "Day")))
-  :set (lambda (symbol value)
-         (set-default symbol value)
-         (setq math-format-date-cache nil)))
-
 (defface calc-nonselected-face
   '((t :inherit shadow
        :slant italic))
@@ -2066,6 +2022,50 @@ See calc-keypad for details."
          (calc-refresh align)))
   (setq calc-refresh-count (1+ calc-refresh-count)))
 
+;; Dates that are built-in options for `calc-gregorian-switch' should be
+;; (YEAR MONTH DAY math-date-from-gregorian-dt(YEAR MONTH DAY)) for speed.
+(defcustom calc-gregorian-switch nil
+  "The first day the Gregorian calendar is used by Calc's date forms.
+This is `nil' (the default) if the Gregorian calendar is the only one used.
+Otherwise, it should be a list `(YEAR MONTH DAY)' when Calc begins to use
+the Gregorian calendar; Calc will use the Julian calendar for earlier dates.
+The dates in which different regions of the world began to use the
+Gregorian calendar vary quite a bit, even within a single country.
+If you want Calc's date forms to switch between the Julian and
+Gregorian calendar, you can specify the date or choose from several
+common choices.  Some of these choices should be taken with a grain
+of salt; for example different parts of France changed calendars at
+different times, and Sweden's change to the Gregorian calendar was
+complicated.  Also, the boundaries of the countries were different at
+the times of the calendar changes than they are now.
+The Vatican decided that the Gregorian calendar should take effect
+on 15 October 1582 (Gregorian), and many Catholic countries made
+the change then.  Great Britain and its colonies had the Gregorian
+calendar take effect on 14 September 1752 (Gregorian); this includes
+the United States."
+  :group 'calc
+  :version "24.4"
+  :type '(choice (const :tag "Always use the Gregorian calendar" nil)
+                 (const :tag "1582-10-15 - Italy, Poland, Portugal, Spain" (1582 10 15 577736))
+                 (const :tag "1582-12-20 - France" (1582 12 20 577802))
+                 (const :tag "1582-12-25 - Luxemburg" (1582 12 25 577807))
+                 (const :tag "1584-01-17 - Bohemia and Moravia" (1584 1 17 578195))
+                 (const :tag "1587-11-01 - Hungary" (1587 11 1 579579))
+                 (const :tag "1700-03-01 - Denmark" (1700 3 1 620607))
+                 (const :tag "1701-01-12 - Protestant Switzerland" (1701 1 12 620924))
+                 (const :tag "1752-09-14 - Great Britain and dominions" (1752 9 14 639797))
+                 (const :tag "1753-03-01 - Sweden" (1753 3 1 639965))
+                 (const :tag "1918-02-14 - Russia" (1918 2 14 700214))
+                 (const :tag "1919-04-14 - Romania" (1919 4 14 700638))
+                 (list :tag "(YEAR MONTH DAY)"
+                       (integer :tag "Year")
+                       (integer :tag "Month (integer)")
+                       (integer :tag "Day")))
+  :set (lambda (symbol value)
+         (set-default symbol value)
+         (setq math-format-date-cache nil)
+         (calc-refresh)))
+
 ;;;; The Calc Trail buffer.
 
 (defun calc-check-trail-aligned ()