S3.1.17-2 and S3.1.17-3, Why have both ?

Why have both (S3.1.17-2) and (S3.1.17-3)? There must be a better way to say (S3.1.17-4) and why have days at all? If this is meant to be cumulative, we need to say something more about that too. Days don't strike me as the same as accumulated wall-clock time. If a consumer encounters acceptable values (including no values) that are outside what interoperable producers produce, what is an implementation expected to do on producing an update of the document?