When a section — one page of a document — last changed: the newest of its own updated_at, the updated_at of the notes its page shows (approved, not archived) and the time a note was archived, which takes the note off the page. A section with no notes keeps its own.

The sitemap's lastmod, the page's dateModified and doc.modified_at, and the "Updated" line under the page's title read this one value. Notes are read as part of the section's own query (latest_note_change_at and latest_note_archived_at), so a batch of sections is one read, and nothing is written to a section when a note changes.

calculate :last_modified_at, :utc_datetime_usec, LastModified