* lisp/calendar/time-date.el (date-days-in-month): Add test for
month validity; signal an error if it isn't. (Bug#40217)
* test/lisp/calendar/time-date-tests.el (test-days-in-month): Add
a test for the new error.
* src/timefns.c (time_arith): Arrange for (time-subtract nil
nil) to yield 0, to be consistent with (time-equal-p nil nil).
* test/lisp/calendar/time-date-tests.el (test-time-since): New test.
* doc/lispref/os.texi (Time Conversion): Document the new
functions that work on decoded time.
(Time Calculations): Document new date/time functions.
* lisp/simple.el (decoded-time-second, decoded-time-minute)
(decoded-time-hour, decoded-time-day, decoded-time-month)
(decoded-time-year, decoded-time-weekday, decoded-time-dst)
(decoded-time-zone): New accessor functions for decoded time values.
* lisp/calendar/time-date.el (date-days-in-month)
(date-ordinal-to-time): New functions.
(decoded-time--alter-month, decoded-time--alter-day)
(decoded-time--alter-second, make-decoded-time): New functions
added to manipulate decoded time structures.
* src/timefns.c (Fdecode_time): Mention the new accessors.
* test/lisp/calendar/time-date-tests.el: New file to test the
decoded time functions and the other new functions.
