The Global Time Zone Database (global-tz) is a fork of the IANA Time
Zone Database with expanded best-effort historical data for some time
zones whose clocks have agreed with other time zones since 1970.

In an up to date default FreeBSD installation /usr/share/zoneinfo
contains the most recent IANA tzdb release as well as compatibility
links for time zones that existed in previous IANA tzdb releases (from
the "backward" file in the distribution).  FreeBSD releases have never
included the out of scope pre-1970 historical data (from the "backzone"
file in the distribution).  This is the same approach as taken by most
other open source projects that distribute the IANA Time Zone Database.

The global-tz fork was prompted by the controversial decision by the
maintainers of the IANA Time Zone Database to move time zones whose
clocks have not changed since 1970 to "backzone".  While previous IANA
tzdb releases would return historical data for these zones if available,
recent IANA tzdb releases instead return available pre-1970 data for the
zones whose clocks they have agreed with since 1970 (the target of the
"backward" link).

The overwhelming majority of users are unlikely to be affected by this
decision.  FreeBSD users who rely on pre-1970 time zone history may wish
to install this port.

This port overwrites system files in /usr/share/zoneinfo.  If you build
your systems from source, add WITHOUT_ZONEINFO="yes" to /etc/src.conf.
If you use freebsd-update(8), add /usr/share/zoneinfo to IgnorePaths in
/etc/freebsd-update.conf.