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, 1 insertions, 1 deletions

diff --git a/doc/misc/emacs-mime.texi b/doc/misc/emacs-mime.texi
index b71cc3755bf..45f37fb8557 100644
--- a/doc/misc/emacs-mime.texi
+++ b/doc/misc/emacs-mime.texi
@@ -1535,7 +1535,7 @@ Here's a bunch of time/date/second/day examples:
 
 @example
 (parse-time-string "Sat Sep 12 12:21:54 1998 +0200")
-@result{} (54 21 12 12 9 1998 6 nil 7200)
+@result{} (54 21 12 12 9 1998 6 -1 7200)
 
 (date-to-time "Sat Sep 12 12:21:54 1998 +0200")
 @result{} (13818 19266)