The time zone is automatically obtained via the network, so if we change the network provider, the correct time zone is also automatically selected. If a different time zone is desired, the automatic time must be deactivated.

Android 10 | ColorOS 7.0
Step 1:  Open the  Settings
Step 2:  Tap on  Additional settings
Step 3:  Tap on  Date & time
Step 4:  Disable  Network-provided time
Step 5:  Tap on  Time zone
Step 6:  Choose desired  Time zone
  1. Open the Settings
  2. Tap on Additional settings
  3. Tap on Date & time
  4. Disable Network-provided time
  5. Tap on Time zone
  6. Choose desired Time zone

Oppo Instructions