tzset.3: Clarify "daylight" and remove erroneous note

When tzset is run the value of daylight is computed
by looking at all available rules for the application
of daylight savings. This includes reading the tzdata
files to determine if there is a transition or not for
the current timezone. It also includes parsing TZ env
to see if it specifies custom rules which are used in
precedence to any tzdata rules. Therefore daylight is
going to be set if there is a daylight saving rule past,
present, or future that indicates a transition. We clarify
that in the man page.

Lastly, the note about tz_dsttime is not correct and is
removed. The earlier paragraph about daylight makes it
clear that it doesn't mean "daylight saving rule applies
now", and the interaction with tz_dsttime is not correct
for glibc on Linux (as outlined in my gettimeofday.3 patch
sent here: http://marc.info/?l=linux-man&m=144977768703615&w=2).

Signed-off-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

diff --git a/man3/tzset.3 b/man3/tzset.3
index 545bd9ccee4ee93422d1862cc3cc53278bc5696a..4786dd9a3c7de3d4f3c334f83d8c5253fa2a0389 100644 (file)
--- a/man3/tzset.3
+++ b/man3/tzset.3
@@ -70,8 +70,8 @@ This function is automatically called by the
 other time conversion functions that depend on the timezone.
 In a System-V-like environment, it will also set the variables \fItimezone\fP
 (seconds West of UTC) and \fIdaylight\fP (to 0 if this timezone does not
-have any daylight saving time rules, or to nonzero if there is a time during
-the year when daylight saving time applies).
+have any daylight saving time rules, or to nonzero if there is a time,
+past, present or future when daylight saving time applies).
 .PP
 If the
 .B TZ
@@ -223,12 +223,6 @@ T} Thread safety   MT-Safe env locale
 .SH CONFORMING TO
 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 .SH NOTES
-Note that the variable \fIdaylight\fP does not indicate that daylight
-saving time applies right now.
-It used to give the number of some
-algorithm (see the variable \fItz_dsttime\fP in
-.BR gettimeofday (2)).
-It has been obsolete for many years but is required by SUSv2.
 .LP
 4.3BSD had a function
 .BI "char *timezone(" zone ", " dst )