Fix ambiguity in nil DST flag

Formerly nil meant both that DST was not in effect and that
the DST flag was unknown, and different functions interpreted
the flag differently.  Now the meaning is consistently nil for
DST not in effect, and -1 for DST flag not known.
* doc/lispref/os.texi (Time Conversion): The DST slot is
now three-valued, not two-.
* doc/misc/emacs-mime.texi (time-date): Adjust to new behavior.
* etc/NEWS: Mention this.
* lisp/calendar/parse-time.el (parse-time-string):
* src/editfns.c (Fdecode_time):
Return -1 for unknown DST flag.
* test/lisp/calendar/parse-time-tests.el (parse-time-tests):
Adjust tests to match new behavior, and add a new
test for nil vs -1.

1 files changed, 11 insertions, 9 deletions

diff --git a/test/lisp/calendar/parse-time-tests.el b/test/lisp/calendar/parse-time-tests.el
index 3a956a56621..9689997f793 100644
--- a/test/lisp/calendar/parse-time-tests.el
+++ b/test/lisp/calendar/parse-time-tests.el
@@ -28,21 +28,23 @@
 
 (ert-deftest parse-time-tests ()
   (should (equal (parse-time-string "Mon, 22 Feb 2016 19:35:42 +0100")
-                 '(42 35 19 22 2 2016 1 nil 3600)))
+                 '(42 35 19 22 2 2016 1 -1 3600)))
   (should (equal (parse-time-string "22 Feb 2016 19:35:42 +0100")
-                 '(42 35 19 22 2 2016 nil nil 3600)))
+                 '(42 35 19 22 2 2016 nil -1 3600)))
   (should (equal (parse-time-string "22 Feb 2016 +0100")
-                 '(nil nil nil 22 2 2016 nil nil 3600)))
+                 '(nil nil nil 22 2 2016 nil -1 3600)))
   (should (equal (parse-time-string "Mon, 22 Feb 16 19:35:42 +0100")
-                 '(42 35 19 22 2 2016 1 nil 3600)))
+                 '(42 35 19 22 2 2016 1 -1 3600)))
   (should (equal (parse-time-string "Mon, 22 February 2016 19:35:42 +0100")
-                 '(42 35 19 22 2 2016 1 nil 3600)))
+                 '(42 35 19 22 2 2016 1 -1 3600)))
   (should (equal (parse-time-string "Mon, 22 feb 2016 19:35:42 +0100")
-                 '(42 35 19 22 2 2016 1 nil 3600)))
+                 '(42 35 19 22 2 2016 1 -1 3600)))
   (should (equal (parse-time-string "Monday, 22 february 2016 19:35:42 +0100")
-                 '(42 35 19 22 2 2016 1 nil 3600)))
-  (should (equal (parse-time-string "Monday, 22 february 2016 19:35:42 PDT")
-                 '(42 35 19 22 2 2016 1 t -25200)))
+                 '(42 35 19 22 2 2016 1 -1 3600)))
+  (should (equal (parse-time-string "Monday, 22 february 2016 19:35:42 PST")
+                 '(42 35 19 22 2 2016 1 nil -28800)))
+  (should (equal (parse-time-string "Friday, 21 Sep 2018 13:47:58 PDT")
+                 '(58 47 13 21 9 2018 5 t -25200)))
   (should (equal (parse-iso8601-time-string "1998-09-12T12:21:54-0200")
                  '(13818 33666)))
   (should (equal (parse-iso8601-time-string "1998-09-12T12:21:54-0230")