To keep your data private within the app, you can enable a password. 

This does not encrypt your data on the hard drive.  If you are using a shared computer, we recommend using separate user profiles for the computer and enabling FileVault to encrypt the hard drive data.

Click Day One > Preferences (in the file menu) > Security

The next time you open Day One, or after the indicated minutes of inactivity, the password prompt will appear.

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