Apple platforms have had the tm_gmtoff-field at least since Mac OS X 10.0,
as are POSIX.1-2024 conforming systems, which are also required to support
it.
This has the unfortunate effect to use the SunOS/Solaris fallback, which
doesn't compute the correct value if the passed value of tm isn't the
current system time, i.e. localtime(::time()) (#3351).