Fix iso8601-parse so unknown DST is -1, not nil

The convention in a decoded time’s dst flag is that t means DST,
nil means standard time, and -1 means unknown.  This differs from
the convention for other components of a decoded time, where nil
means unknown.  Fix some places where iso8601-parse mistakenly
treated nil as meaning that the dst flag was unknown.
* doc/lispref/os.texi (Time Parsing):
Adjust to match parse-time-string’s doc string.
* lisp/calendar/iso8601.el (iso8601-parse):
Set dst flag to nil if a numeric time zone or "Z" is given.
(iso8601--decoded-time): Default dst flag to -1 if no dst
flag or zone is given.
* lisp/calendar/time-date.el (decoded-time-set-defaults):
When we don’t have a time zone, set the dst flag consistently
with DEFAULT-ZONE.
* test/lisp/calendar/iso8601-tests.el (test-iso8601-date-years)
(test-iso8601-date-dates, test-iso8601-date-obsolete)
(test-iso8601-date-weeks, test-iso8601-date-ordinals)
(test-iso8601-time, test-iso8601-combined)
(test-iso8601-duration, test-iso8601-intervals)
(standard-test-dates, standard-test-time-of-day-local-time)
(standard-test-time-of-day-fractions)
(nonstandard-test-time-of-day-decimals)
(standard-test-time-of-day-beginning-of-day)
(standard-test-date-and-time-of-day, standard-test-interval):
Adjust tests to match fixed behavior.

2 files changed, 14 insertions, 11 deletions

diff --git a/lisp/calendar/iso8601.el b/lisp/calendar/iso8601.el
index ae1dab17252..e42fe0fa21f 100644
--- a/lisp/calendar/iso8601.el
+++ b/lisp/calendar/iso8601.el
@@ -136,7 +136,8 @@ See `decode-time' for the meaning of FORM."
       (when zone-string
         (setf (decoded-time-zone date)
               ;; The time zone in decoded times are in seconds.
-              (* (iso8601-parse-zone zone-string) 60)))
+              (* (iso8601-parse-zone zone-string) 60))
+        (setf (decoded-time-dst date) nil))
       date)))
 
 (defun iso8601-parse-date (string)
@@ -332,6 +333,9 @@ Return the number of minutes."
     (list start end
           (or duration
               ;; FIXME: Support subseconds.
+              ;; FIXME: It makes no sense to decode a time difference
+              ;; according to (decoded-time-zone end), or according to
+              ;; any other time zone for that matter.
               (decode-time (time-subtract (iso8601--encode-time end)
                                           (iso8601--encode-time start))
                            (or (decoded-time-zone end) 0) 'integer)))))
@@ -354,7 +358,7 @@ Return the number of minutes."
         (iso8601--value month)
         (iso8601--value year)
         nil
-        dst
+        (if (or dst zone) dst -1)
         zone))
 
 (defun iso8601--encode-time (time)
diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index 1e589ece29d..e2402de8010 100644
--- a/lisp/calendar/time-date.el
+++ b/lisp/calendar/time-date.el
@@ -515,15 +515,14 @@ TIME is modified and returned."
   (unless (decoded-time-year time)
     (setf (decoded-time-year time) 0))
 
-  ;; When we don't have a time zone and we don't have a DST, then mark
-  ;; it as unknown.
-  (when (and (not (decoded-time-zone time))
-             (not (decoded-time-dst time)))
-    (setf (decoded-time-dst time) -1))
-
-  (when (and (not (decoded-time-zone time))
-             default-zone)
-    (setf (decoded-time-zone time) 0))
+  ;; When we don't have a time zone, default to DEFAULT-ZONE without
+  ;; DST if DEFAULT-ZONE if given, and to unknown DST otherwise.
+  (unless (decoded-time-zone time)
+    (if default-zone
+        (progn (setf (decoded-time-zone time) default-zone)
+               (setf (decoded-time-dst time) nil))
+      (setf (decoded-time-dst time) -1)))
+
   time)
 
 (provide 'time-date)