WikibaseDate exclusively handles the UTC time zone, i.e., the Z character (cf. W3C specs):
https://github.com/wikimedia/wikidata-query-rdf/blob/master/common/src/main/java/org/wikidata/query/rdf/common/WikibaseDate.java#L30
This may conflict with data serialized through other RDF libraries.
For instance, Python rdflib automatically translates Z into +00:00, which prevents proper ingestion into Blazegraph, due to:
https://github.com/wikimedia/wikidata-query-rdf/blob/master/common/src/main/java/org/wikidata/query/rdf/common/WikibaseDate.java#L46