Research Unix, 7th Edition introduced TIMEZONE and DSTFLAG
compile-time constants in sys/param.h to communicate these values for
the machine. 4.2BSD moved from the compile-time to run-time and
introduced these variables and used for localtime() to return the
right offset from UTC (sometimes referred to as GMT, for this purpose
is the same). 4.4BSD migrated to using the Olsen code/database and
these variables were basically unused.
FreeBSD removed the real need for these with adjkerntz in 1995.
However, some RTC clocks continued to use these variables,
though they were largely unused otherwise. Later, phk centeralized
most of the uses in utc_offset, but left it using both tz_minuteswest
POSIX (IEEE Std 1003.1-2017) states in the gettimeofday specification
"If tzp is not a null pointer, the behavior is unspecified" so there's
no standards reason to retain it anymore. In fact, gettimeofday has
been marked as obsolecent, meaning it could be removed from a future
release of the standard. It is the only interface defined in POSIX
that references these two values. All other references come from the
olson database via tzset().
These were used to more faithfully implement early unix ABIs which
have been removed from FreeBSD. NetBSD has completely eliminated
these variables. Linux has migrated to Olson as well, though these
variables technically still exist for standards conformance. They
appear to always be 0.
So, there's no real reason to have them these days. They are a
historical vestige that's no longer used in any meaningful way.
Remove now useless -d and -t flags in date(1)
These were used to set dst flag and minutes west of UTC
respectively. These are obsolete and have been removed form the
kernel. These existed primarily to faithfully emulate early
Unix ABIs that have been removed from FreeBSD.