The journal
Today has a page.
Days with an entry get a mark. Tap one to read it back.
Entries are never edited here. The page you wrote stays exactly as you wrote it. Notes get their own date instead.
The export is one plain file holding the journal and the check-in. Save it to iCloud Drive and it is backed up from then on, still there when you get a new phone. Importing merges rather than replaces: an older file cannot wipe a fuller journal.
We do not receive any of this, store it, or have any way to read it. Because we keep no copy, there is no copy: the export is the backup, and the privacy page walks through it.