Forgot Password on Mac:
If you lost the password on the Mac app, you can follow these steps:
Open the journal file location in Finder. To do so: Open Day One, and open the Preferences window (Day One > Preferences) Click on the “Sync” tab. Click on the path under the “Data File” title. This will open a Finder window.
In Finder, find the “security.plist” file. To find it:If you see “Journal.dayone”, right click it and select “Show Package Contents”If you see “Journal_dayone” then click the disclosure arrow to show the contentsOnce you’ve found the “security.plist” file, move it to the Trash.Restart Day One.
For Day One 2.0, use this guide instead: http://help.dayoneapp.com/day-one-2-0/resetting-day-one-2xs-password
Forgot Passcode on iPhone / iPad:
The easiest solution is to try and remember, maybe spend a while guessing possibly combinations.
The next easiest solution only works if you are certain that your data has been synced to Dropbox, iCloud, or Day One Sync. 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.