The OnePlus smartphone automatically gets the time zone from the network.

If the time zone or region is to be changed afterwards, the automatic function must first be deactivated.
After selecting the desired region, the corresponding time zone is then set.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Settings
Step 2:  Tap on  System
Step 3:  Tap on  Date & time
Step 4:  Disable  Use network-provided time zone
Step 5:  Tap on  Time zone
Step 6:  Tap on  Region (country/region)
Step 7:  Select a  country  or enter a  name
Step 8:  Chosse a  suggestion
Step 9:  Choose a  time zone
  1. Open the Settings
  2. Tap on System
  3. Tap on Date & time
  4. Disable Use network-provided time zone
  5. Tap on Time zone
  6. Tap on Region (country/region)
  7. Select a country or enter a name
  8. Chosse a suggestion
  9. Choose a time zone

OnePlus Instructions