The date is automatically obtained and set by the OnePlus smartphone from the network.

If the date is to be set manually for personal reasons, the automatic function must first 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:  Disable  Use network-provided time
Step 5:  Tap on  Date
Step 6:  Set the  date  and tap  OK
  1. Open the Settings
  2. Tap on System
  3. Tap on Date & time
  4. Disable Use network-provided time
  5. Tap on Date
  6. Set the date and tap OK

OnePlus Instructions