The precision of a TimeValue is currently represented as a nonNegativeInteger in RDF. See docs/ontology.owl, recently updated in https://gerrit.wikimedia.org/r/#/c/207632. But these numbers do not have a meaning. They are just constants, for internal use in the Wikibase code base. In RDF it probably makes much more sense to represent these precision values as URIs, each with a well defined meaning.
See the definition of the precisions here: https://www.mediawiki.org/wiki/Wikibase/DataModel#Dates_and_times