Instructions for iOS and macOS below.
iOS
Go to Day One > Settings > Journals > tap the Journal for which you need to locate the ID > Advanced > Journal ID. Tap the journal ID to copy to the clipboard.
macOS
Go to Day One > Preferences > Journals > click the “i” icon to the right of the journal. Locate the journal ID at the bottom left of the journal information details.
Android
Go to the Server Dashboard at https://dayone.app and sign in. The journal ID is listed below the journal name.