Set the local time zone using TZ, a string specifying a time zone rule.
If TZ is nil, use implementation-defined default time zone information.
If TZ is t, use Universal Time.
Instead of calling this function, you typically want (setenv "TZ" TZ).
That changes both the environment of the Emacs process and the
variable `process-environment', whereas `set-time-zone-rule' affects
only the former. (fn TZ)