If you have tried moving entries from Day One to Apple’s Journal app en masse, you have no doubt hit a wall. Journal has no AppleScript support, no public API, and very limited Shortcuts support.
I assumed this would remain unsolved until Apple created an importer for Journal, but then I saw Brian Stucki link to this project on GitHub.
I made an extra backup of my Day One data and then threw nearly 3,000 Day One posts at it, and the CLI just worked.