date(1) does not include timezone in some cases

[wezm]

The date(1) man page says that the timezone should be included when passing -Iseconds or -R but I'm always getting no timezone:

# Chimera
$ date -Iseconds
2023-07-15T09:34:37

$ ls -ld /etc/localtime
lrwxrwxrwx  1 root  root  38 Jun 15 10:45 /etc/localtime -> /usr/share/zoneinfo/Australia/Brisbane

I confirmed it works as documented on a FreeBSD 13.2 system:

#FreeBSD
$ date -Iseconds
2023-07-15T09:32:49+10:00

For -I this is how date attempts to get the time zone:

chimerautils/src.freebsd/coreutils/date/date.c

Lines 241 to 246 in eb01fbc

	if (iso8601_selected > iso8601_fmts) {
	(void)strftime_bsd(tzbuf, sizeof(tzbuf), "%z", lt);
	memmove(&tzbuf[4], &tzbuf[3], 3);
	tzbuf[3] = ':';
	strlcat(buf, tzbuf, sizeof(buf));
	}

When strftime_bsd processes %z it does a number of checks:

The first is:

chimerautils/src.freebsd/compat/strftime.c

Line 457 in eb01fbc

if (t->tm_isdst < 0)

In my testing localtime() is returning a struct tm with tm_isdst set to 0, so it should get past this condition and into this:

chimerautils/src.freebsd/compat/strftime.c

Lines 459 to 493 in eb01fbc

	#ifdef TM_GMTOFF
	diff = t->TM_GMTOFF;
	#else /* !defined TM_GMTOFF */
	/*
	* C99 says that the UTC offset must
	* be computed by looking only at
	* tm_isdst. This requirement is
	* incorrect, since it means the code
	* must rely on magic (in this case
	* altzone and timezone), and the
	* magic might not have the correct
	* offset. Doing things correctly is
	* tricky and requires disobeying C99;
	* see GNU C strftime for details.
	* For now, punt and conform to the
	* standard, even though it's incorrect.
	*
	* C99 says that %z must be replaced by the
	* empty string if the time zone is not
	* determinable, so output nothing if the
	* appropriate variables are not available.
	*/
	if (t->tm_isdst == 0)
	#ifdef USG_COMPAT
	diff = -timezone;
	#else /* !defined USG_COMPAT */
	continue;
	#endif /* !defined USG_COMPAT */
	else
	#ifdef ALTZONE
	diff = -altzone;
	#else /* !defined ALTZONE */
	continue;
	#endif /* !defined ALTZONE */
	#endif /* !defined TM_GMTOFF */

It's unclear to me which of the compile time definitions are set. It seems than in FreeBSD these are set in tzconfig.h. It seems that that code was not imported in 65d666e, which might mean that all of them are unset, which looks like it would fall though all the tests and skip over %z resulting in the observed behaviour.