This document is clear for its scope. It simply adds new type definitions to the "ietf-yang-types" and "ietf-inet-types" YANG modules and obsoletes RFC 6991.

The new types defined in the YANG modules are quite understandable, but I would suggest to add some explanation, maybe in section 2, about the motivations behind the addition of these new types (for example, the new date/time related types compared to the date-and-time type already defined in RFC 6021).

I noticed that there are two appendixes about the changes from RFC 6991 and from RFC 6021, which only refer to section 3 and section 4. I think it is useful to add a reference also to section 2, since the tables there show the new types with respect to RFC 6991 and RFC 6021. Additionally, you can consider to move these appendixes as subsections of section 1.