Automatic aggregation: Day → Week → Month → Quarter → Year.
Notes roll up automatically along the calendar spine — day into week, week into month, up to the year — so summaries stay current without manual effort.
Everyone intends to review. Almost nobody does, because reviewing means reconstructing — scrolling back through weeks, remembering what mattered, assembling a picture from scratch at exactly the moment you have least energy for it. The intention isn't the problem; the reconstruction cost is.
Roll-ups remove the cost by making each level a summary of the one below. The week reads the days, the month reads the weeks, the year reads the months. Nothing is re-derived from raw material — each layer only has to summarise a layer that's already been summarised.
Which is why it only works on a spine (the calendar). One predictable axis means every level knows exactly what to read. Cleverness fragments; a spine cascades.