This patch is in response to Eli's review (Bug#32902#10).
* src/systime.c: Doc strings of affected functions now refer
to format-time-string instead of to Lisp manual, and
format-time-string's doc string covers time values.
* test/src/systime-tests.el (format-time-string-with-zone):
Check decode-time too.
(decode-then-encode-time, time-arith-tests): New tests.
3cc452327e