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

[Bob Rogers]

   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))