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.

Mac
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.

iOS/Android
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.
- 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.