TZ variable can be set on on for a session, user profile or use it in the Database start-up script to start the database for the specific timezone or country or city.
Following are some examples can be tested in a terminal session of UNIX.
For China is UTC+8:00 or GMT+8:00
TZ needs to set it with opposite sign as follows:
TZ=UTC-8:00; export TZ
TZ=GMT-8:00; export TZ
setenv("TZ", "GMT-8:00", 1) tzset();
For US EST (GMT-5:00, New York):
TZ=UTC+5:00; export TZ
TZ=GMT+5:00; export TZ
setenv("TZ", "UTC+5:00", 1) tzset();