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, 3 insertions, 2 deletions

diff --git a/src/bignum.c b/src/bignum.c
index 3883d3a3944..90b1ebea876 100644
--- a/src/bignum.c
+++ b/src/bignum.c
@@ -31,9 +31,10 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
    storage is exhausted.  Admittedly this is not ideal.  An mpz value
    in a temporary is made permanent by mpz_swapping it with a bignum's
    value.  Although typically at most two temporaries are needed,
-   time_arith, rounddiv_q and rounding_driver each need four.  */
+   rounddiv_q and rounding_driver both need four and time_arith needs
+   five.  */
 
-mpz_t mpz[4];
+mpz_t mpz[5];
 
 static void *
 xrealloc_for_gmp (void *ptr, size_t ignore, size_t size)