--- Begin Message ---
Subject: |
30.0.50; [PATCH; DOC] time-to-days and days-to-time use different epochs |
Date: |
Thu, 12 Oct 2023 10:23:35 -0700 |
Imagine my surprise when converting back using days-to-time gave me
something almost two millenia in the future. Of course, the problem was
that days-to-time uses a different epoch from time-to-days. The fix was
pretty easy . . . and then I discovered all the code that depends on the
existing behavior. So I offer the following modest documentation patch,
in the hope that others will be spared any similar confusion.
-- Bob Rogers
http://www.rgrjr.com/
diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index 5400d492f0a..2f6bb4e190c 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -2127,6 +2127,14 @@ Time Calculations
The operating system limits the range of time and zone values.
@end defun
+@defun days-to-time days
+This is not quite the inverse of the @code{time-to-days} function, as
+it uses the Emacs epoch (instead of the year 1) for historical
+reasons. To get the inverse, subtract @code{(time-to-days 0)} from
+@var{days}, in which case @code{days-to-time} may return @code{nil} if
+@var{days} is negative.
+@end defun
+
@defun time-to-day-in-year time-value
This returns the day number within the year corresponding to @var{time-value},
assuming the default time zone.
diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index 9cbe8e0f53c..786134d8ac5 100644
--- a/lisp/calendar/time-date.el
+++ b/lisp/calendar/time-date.el
@@ -181,7 +181,10 @@ seconds-to-time
;;;###autoload
(defun days-to-time (days)
- "Convert DAYS into a time value."
+ "Convert Emacs-epoch DAYS into a time value.
+Note that this does not use the same epoch as time-to-days; you
+must subtract (time-to-days 0) first to convert, and may get nil
+if the result is before the start."
;; FIXME: We should likely just pass `t' to `time-convert'.
;; All uses I could find in Emacs, GNU ELPA, and NonGNU ELPA can handle
;; any valid time representation as return value.
@@ -243,7 +246,7 @@ time-to-day-in-year
;;;###autoload
(defun time-to-days (time)
- "The absolute date corresponding to TIME, a time value.
+ "The absolute pseudo-Gregorian date for TIME, a time value.
The absolute date is the number of days elapsed since the imaginary
Gregorian date Sunday, December 31, 1 BC."
(let* ((tim (decode-time time))
--- End Message ---
--- Begin Message ---
Subject: |
Re: bug#66502: 30.0.50; [PATCH; DOC] time-to-days and days-to-time use different epochs |
Date: |
Sat, 14 Oct 2023 12:02:10 +0300 |
> From: Bob Rogers <rogers@rgrjr.com>
> Date: Thu, 12 Oct 2023 10:23:35 -0700
>
> Imagine my surprise when converting back using days-to-time gave me
> something almost two millenia in the future. Of course, the problem was
> that days-to-time uses a different epoch from time-to-days. The fix was
> pretty easy . . . and then I discovered all the code that depends on the
> existing behavior. So I offer the following modest documentation patch,
> in the hope that others will be spared any similar confusion.
Thanks, installed on the emacs-29 branch, and closing the bug.
Please in the future accompany the patches by a ChangeLog-style commit
log message, it makes the patches easier to install.
--- End Message ---