doc: Mention a gmtime_r problem

[Bruno Haible]

POSIX specifies that gmtime_r sets the tm_zone field to "UTC". (Interestingly,
no such requirement exists for gmtime.) The comments in tests/test-gmtime_r.c
remind us that this is not always the case.


2024-09-03  Bruno Haible  <bruno@clisp.org>

        doc: Mention a gmtime_r problem.
        * doc/posix-functions/gmtime_r.texi: Mention "UTC" vs. "GMT".

diff --git a/doc/posix-functions/gmtime_r.texi 
b/doc/posix-functions/gmtime_r.texi
index f49547595b..6474b9151a 100644
--- a/doc/posix-functions/gmtime_r.texi
+++ b/doc/posix-functions/gmtime_r.texi
@@ -23,4 +23,8 @@
 On some platforms, this function yields incorrect values for
 timestamps before the year 1:
 MacOS X 10.5, Solaris 11.3.
+@item
+This function sets the field @code{tm_zone} to @code{"GMT"}
+instead of @code{"UTC"} on some platforms:
+glibc, NetBSD, OpenBSD, Minix, Cygwin, Android.
 @end itemize