It is not incorrect in general, but it is kind of hack that may be incorrect in some situations (as it does not reflect change in system timezone), so it is not suitable for default behavior.
Default behavior - use current system timezone
Explicit TZ - use specific timezone defined by TZ variable, defined either directly (e.g. TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"), or indirectly from file (e.g. TZ=":/usr/share/zoneinfo/Europe/Brussels"). As this defines specific timezone, it is not supposed to change.
