The date and time are automatically obtained by the OnePlus smartphone from the provider network and require no further setting.

Note: If the date or time is to be set manually, the automatic function must be deactivated.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Settings
Step 2:  Tap on  System
Step 3:  Tap on  Date & time
Step 4:  Enable  Use network-provided time
  1. Open the Settings
  2. Tap on System
  3. Tap on Date & time
  4. Enable Use network-provided time

OnePlus Instructions