On the sum nobody adds up
“How we spend our days is, of course, how we spend our lives.”
Of course is the whole sentence. Dillard states an identity so obvious that it has never once been acted on, and the little aside admits that everyone already knew.
The reason it does not land is scale. A life is imagined as a shape to be arranged later; a day is felt as something to be got through. Nothing connects the two in ordinary experience, and Dillard's sentence is the connection, offered without any comfort attached.