Day One Mac
The Mac version of Day One allows you to set a private security passcode. For optimal security, it is important to have a private user login to your Mac (that you do not share with others) if you are sharing a computer. Read macOS: Setting Up User Accounts for more information on setting up individual accounts on your Mac.
Day One iOS
Like the Mac version, you can enable a Day One passcode to restrict usage to yourself or those with whom you share the passcode. Follow these steps:
Open Day One.
Tap Settings, then tap the Passcode/Touch ID.
Enter and verify your passcode.
By default, your passcode will be required whenever you leave Day One or your device goes to sleep. Tap "Immediately" to choose other frequency options for your passcode.
To encrypt your data on your iOS device, you can enable a passcode for your device (this is different from the preceding Day One passcode). To do this, follow these steps:
Go to Settings > General.
Tap "Turn Passcode On".
For more information, read Apple's support article, Understanding Data Protection.