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, 22 insertions, 22 deletions

diff --git a/test/src/timefns-tests.el b/test/src/timefns-tests.el
index 48d964d129c..3a18a4a24dd 100644
--- a/test/src/timefns-tests.el
+++ b/test/src/timefns-tests.el
@@ -19,6 +19,12 @@
 
 (require 'ert)
 
+(defun timefns-tests--decode-time (look zone decoded-time)
+  (should (equal (decode-time look zone t) decoded-time))
+  (should (equal (decode-time look zone 'integer)
+                 (cons (time-convert (car decoded-time) 'integer)
+                       (cdr decoded-time)))))
+
 ;;; Check format-time-string and decode-time with various TZ settings.
 ;;; Use only POSIX-compatible TZ values, since the tests should work
 ;;; even if tzdb is not in use.
@@ -40,31 +46,29 @@
                     (7879679999900 . 100000)
                     (78796799999999999999 . 1000000000000)))
       ;; UTC.
-     (let ((sec (time-add 59 (time-subtract (time-convert look t)
-                                            (time-convert look 'integer)))))
+     (let* ((look-ticks-hz (time-convert look t))
+            (hz (cdr look-ticks-hz))
+            (look-integer (time-convert look 'integer))
+            (sec (time-add (time-convert 59 hz)
+                           (time-subtract look-ticks-hz
+                                          (time-convert look-integer hz)))))
       (should (string-equal
                (format-time-string "%Y-%m-%d %H:%M:%S.%3N %z" look t)
                "1972-06-30 23:59:59.999 +0000"))
-      (should (equal (decode-time look t 'integer)
-                     '(59 59 23 30 6 1972 5 nil 0)))
-      (should (equal (decode-time look t t)
-                     (list sec 59 23 30 6 1972 5 nil 0)))
+      (timefns-tests--decode-time look t
+                                  (list sec 59 23 30 6 1972 5 nil 0))
       ;; "UTC0".
       (should (string-equal
                (format-time-string format look "UTC0")
                "1972-06-30 23:59:59.999 +0000 (UTC)"))
-      (should (equal (decode-time look "UTC0" 'integer)
-                     '(59 59 23 30 6 1972 5 nil 0)))
-      (should (equal (decode-time look "UTC0" t)
-                     (list sec 59 23 30 6 1972 5 nil 0)))
+      (timefns-tests--decode-time look "UTC0"
+                                  (list sec 59 23 30 6 1972 5 nil 0))
       ;; Negative UTC offset, as a Lisp list.
       (should (string-equal
                (format-time-string format look '(-28800 "PST"))
                "1972-06-30 15:59:59.999 -0800 (PST)"))
-      (should (equal (decode-time look '(-28800 "PST") 'integer)
-                     '(59 59 15 30 6 1972 5 nil -28800)))
-      (should (equal (decode-time look '(-28800 "PST") t)
-                     (list sec 59 15 30 6 1972 5 nil -28800)))
+      (timefns-tests--decode-time look '(-28800 "PST")
+                                  (list sec 59 15 30 6 1972 5 nil -28800))
       ;; Negative UTC offset, as a Lisp integer.
       (should (string-equal
                (format-time-string format look -28800)
@@ -73,18 +77,14 @@
                (if (eq system-type 'windows-nt)
                    "1972-06-30 15:59:59.999 -0800 (ZZZ)"
                  "1972-06-30 15:59:59.999 -0800 (-08)")))
-      (should (equal (decode-time look -28800 'integer)
-                     '(59 59 15 30 6 1972 5 nil -28800)))
-      (should (equal (decode-time look -28800 t)
-                     (list sec 59 15 30 6 1972 5 nil -28800)))
+      (timefns-tests--decode-time look -28800
+                                  (list sec 59 15 30 6 1972 5 nil -28800))
       ;; Positive UTC offset that is not an hour multiple, as a string.
       (should (string-equal
                (format-time-string format look "IST-5:30")
                "1972-07-01 05:29:59.999 +0530 (IST)"))
-      (should (equal (decode-time look "IST-5:30" 'integer)
-                     '(59 29 5 1 7 1972 6 nil 19800)))
-      (should (equal (decode-time look "IST-5:30" t)
-                     (list sec 29 5 1 7 1972 6 nil 19800)))))))
+      (timefns-tests--decode-time look "IST-5:30"
+                                  (list sec 29 5 1 7 1972 6 nil 19800))))))
 
 (ert-deftest decode-then-encode-time ()
   (let ((time-values (list 0 -2 1 0.0 -0.0 -2.0 1.0