* lisp/calendar/iso8601.el (iso8601--zone-dst): New function.
(iso8601-parse, iso8601-parse-time): Use it.
(iso8601--decoded-time): Default dst to -1, not nil.
* test/lisp/calendar/iso8601-tests.el (test-iso8601-combined)
(standard-test-time-of-day-zone): Adjust to new behavior.