Access to Day One can be protected with an app-locking password. Keep in mind, this password does not sync between devices. It can be a different password for each device.
The app-locking password for Day One on Mac is securely stored in the Keychain Access app.
Quit Day One
Open Keychain Access*.
Click the section for "Login" in the left panel.
Search for "day one".
Control-click (or right-click) Day One App Locking Password to reveal a contextual menu.
Select Delete “Day One App Locking Password”.
Return to Day One to open it without your previously-set password.
*Keychain Access can be found by searching Spotlight or going to Finder > Applications > Utilities > Keychain Access.
There are 3 possible options to reset the app-locking passcode on iOS.
The easiest solution is to try and remember the passcode. Maybe spend a while guessing possible combinations.
If you have Touch ID/Face ID or an Android fingerprint enabled for Day One but can't remember the passcode, here are the steps to remove the passcode:
Go to Day One > Settings > Passcode & Biometrics
Turn off the Passcode.
When the numberpad comes up, tap the Touch ID/Face ID icon in the bottom left corner.
Enter your fingerprint or use Face ID. The passcode will now be reset.
3. The last solution only works if you are certain that your data has been synced to Day One Sync. Verify by signing in to the Server Dashboard (https://dayone.app) in a web browser. If that’s the case, you can delete the app from the device and re-install it. Then re-sync your data from the previous sync method.