Oppo
Set Date Manually auf Oppo
On Oppo phones running ColorOS, the date field stays locked as long as automatic time sync is active. The system pulls date and time from the mobile network or GPS, which is why there is no separate switch to edit only the date.
To change it manually, you first have to disable the automatic source. Once that is off, ColorOS unlocks both date and time fields for manual input, so it makes sense to check the time as well while you are in that screen.
Open Tap on Settings ➔ Tap on Additional settings ➔ Tap on Date & time ➔ Turn Off Set automatically. With that toggle off, tap Tap on Date and Set the Date to the correct day, month and year.
Leaving automatic sync off has a side effect: the phone will no longer correct itself after a time zone change or daylight saving shift. If you travel often or connect to different networks, it is usually better to keep automatic sync on and only switch it off temporarily when the date is clearly wrong, for example after a battery drained to zero.