Avoid some excess precision in time arithmetic

* doc/misc/emacs-mime.texi (time-date):
Adjust example to match new behavior.
* etc/NEWS: Mention this.
* lisp/calendar/time-date.el (decoded-time-add)
(decoded-time--alter-second):
Don’t lose underestimate precision of seconds component.
* src/bignum.c (mpz): Grow by 1.
* src/timefns.c (trillion_factor): New function.
(timeform_sub_ps_p): Remove.
(time_arith): Avoid unnecessarily-large hz, by reducing the hz
to a value no worse than the worse hz of the two arguments.
The result is always exact unless an error is signaled.
* test/src/timefns-tests.el (timefns-tests--decode-time):
New function.
(format-time-string-with-zone): Test (decode-time LOOK ZONE t)
resolution as well as its numeric value.

1 files changed, 21 insertions, 11 deletions

diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index f3d252f03c6..11bd469ae3b 100644
--- a/lisp/calendar/time-date.el
+++ b/lisp/calendar/time-date.el
@@ -421,10 +421,13 @@ changes in daylight saving time are not taken into account."
     ;; Do the time part, which is pretty simple (except for leap
     ;; seconds, I guess).
     ;; Time zone adjustments are basically the same as time adjustments.
-    (setq seconds (time-add (+ (* (or (decoded-time-hour delta) 0) 3600)
-                               (* (or (decoded-time-minute delta) 0) 60)
-                               (or (decoded-time-zone delta) 0))
-                            (or (decoded-time-second delta) 0)))
+    (setq seconds (time-convert (or (decoded-time-second delta) 0) t))
+    (setq seconds
+          (time-add seconds
+                    (time-convert (+ (* (or (decoded-time-hour delta) 0) 3600)
+                                     (* (or (decoded-time-minute delta) 0) 60)
+                                     (or (decoded-time-zone delta) 0))
+                                  (cdr seconds))))
 
     (decoded-time--alter-second time seconds)
     time))
@@ -461,11 +464,16 @@ changes in daylight saving time are not taken into account."
 
 (defun decoded-time--alter-second (time seconds)
   "Increase the time in TIME by SECONDS."
-  (let* ((secsperday 86400)
-         (old (time-add (+ (* 3600 (or (decoded-time-hour time) 0))
-                           (* 60 (or (decoded-time-minute time) 0)))
-                        (or (decoded-time-second time) 0)))
-         (new (time-add old seconds)))
+  (let* ((time-sec (time-convert (or (decoded-time-second time) 0) t))
+         (time-hz (cdr time-sec))
+         (old (time-add time-sec
+                        (time-convert
+                         (+ (* 3600 (or (decoded-time-hour time) 0))
+                            (* 60 (or (decoded-time-minute time) 0)))
+                         time-hz)))
+         (new (time-convert (time-add old seconds) t))
+         (new-hz (cdr new))
+         (secsperday (time-convert 86400 new-hz)))
     ;; Hm...  DST...
     (while (time-less-p new 0)
       (decoded-time--alter-day time nil)
@@ -474,8 +482,10 @@ changes in daylight saving time are not taken into account."
       (decoded-time--alter-day time t)
       (setq new (time-subtract new secsperday)))
     (let ((sec (time-convert new 'integer)))
-      (setf (decoded-time-second time) (time-add (% sec 60)
-                                                 (time-subtract new sec))
+      (setf (decoded-time-second time) (time-add
+                                        (time-convert (% sec 60) new-hz)
+                                        (time-subtract
+                                         new (time-convert sec new-hz)))
             (decoded-time-minute time) (% (/ sec 60) 60)
             (decoded-time-hour time) (/ sec 3600)))))