Sunday in The Trivium

This sentence came as a surprise:

When is position in relation to the course of extrinsic events which measure the duration of a substance, for example, Sunday afternoon.

Sister Miriam Joseph, The Trivium: The Liberal Arts of Logic, Grammar, and Rhetoric, ed. Marguerite McGlinn (Philadelphia: Paul Dry Books 2002).

[When, or time, is one of Aristotle’s ten categories of being.]