Well, in their defense, when they were writing the document there was no standard yet

It’s just a standard specification. The standard specification doesn’t specify which standards it itself should use. That will have been done (and should be done) in a different document. Imagine if each standard specification also references all the standards it employs. That would lead to unnecessarily bulky documents.
Additionally, the keywords informatiemanagement and metadata indicate that this standard is intended for use in information management and metadata settings. Not necessarily applicable to a web front-end setting

From a programming Perspective:

They probably use a JS Library that automatically converts the Date to the Format that is the default in your Region. Try it out with a VPN in the US or in Japan and you’ll see different Dates I’d bet.