GNU bug report logs - #66502
30.0.50; [PATCH; DOC] time-to-days and days-to-time use different epochs

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#66502: closed (30.0.50; [PATCH; DOC] time-to-days and
 days-to-time use different epochs)
Date: Sat, 14 Oct 2023 09:03:02 +0000

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Your message dated Sat, 14 Oct 2023 12:02:10 +0300
with message-id <835y39shkd.fsf <at> gnu.org>
and subject line Re: bug#66502: 30.0.50; [PATCH; DOC] time-to-days and days-to-time use different epochs
has caused the debbugs.gnu.org bug report #66502,
regarding 30.0.50; [PATCH; DOC] time-to-days and days-to-time use different epochs
to be marked as done.

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From: Bob Rogers <rogers <at> rgrjr.com>
To: bug-gnu-emacs <at> gnu.org
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

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   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/

[rgr-days-to-time-doc-1.patch (text/x-patch, inline)]

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))

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Bob Rogers <rogers <at> rgrjr.com>
Cc: 66502-done <at> debbugs.gnu.org
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 <at> 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.

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