(Time): In strftime, note systems vary for %Z.

doc/ref/posix.texi

@@ -1230,6 +1230,28 @@ formatting.
 
 If @code{setlocale} has been called (@pxref{Locales}), month and day
 names are from the current locale and in the locale character set.
+
+Note that @samp{%Z} might print the @code{tm:zone} in @var{tm} or it
+might print just the current zone (@code{tzset} above).  A GNU system
+prints @code{tm:zone}, a strict C99 system like NetBSD prints the
+current zone.  Perhaps in the future Guile will try to get
+@code{tm:zone} used always.
+@c
+@c  The issue in the above is not just whether tm_zone exists in
+@c  struct tm, but whether libc feels it should read it.  Being a
+@c  non-C99 field, a strict C99 program won't know to set it, quite
+@c  likely leaving garbage there.  NetBSD, which has the field,
+@c  therefore takes the view that it mustn't read it.  See the PR
+@c  about this at
+@c
+@c      http://www.netbsd.org/cgi-bin/query-pr-single.pl?number=21722
+@c
+@c  Uniformly making tm:zone used on all systems (all those which have
+@c  %Z at all of course) might be nice (either mung TZ and tzset, or
+@c  mung tzname[]).  On the other hand it would make us do more than
+@c  C99 says, and we really don't want to get intimate with the gory
+@c  details of libc time funcs, no more than can be helped.
+@c
 @end deffn
 
 @deffn {Scheme Procedure} strptime format string