That depends on your application(s). E.g. databases tend to crash when an operation finishes before it started. That would happen if your server is running ahead of time and you set it back during a DB operation is running. If your server's time is behind real time you may simply
# smitty date
for setting date and time and
# smitty chtz
for setting TZ
If you are unsure change both settings only with your apps stopped.