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, 2 insertions, 1 deletions

diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index 0b9dd1c9cc3..43ca9ede00b 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -1423,7 +1423,8 @@ The year, an integer typically greater than 1900.
 The day of week, as an integer between 0 and 6, where 0 stands for
 Sunday.
 @item dst
-@code{t} if daylight saving time is effect, otherwise @code{nil}.
+@code{t} if daylight saving time is effect, @code{nil} if it is not
+in effect, and @minus{}1 if this information is not available.
 @item utcoff
 An integer indicating the Universal Time offset in seconds, i.e., the number of
 seconds east of Greenwich.