tzset ¶POSIX specification:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/tzset.html
Portability problems fixed by Gnulib:
TZ has been set by Cygwin.
Portability problems not fixed by Gnulib:
TZ environment variable,
consisting of a time zone abbreviation containing exactly three ASCII
letters with no daylight saving time or angle brackets, and with no
support for tz database settings like
TZ='America/New_York'. Even this subset does not work on
applications built via the Universal Windows Platform, as it does not
make environment variables like TZ available to applications.
TZ values,
as this feature was added in IEEE Std 1003.1-2001.