Ordering results by date types in the public SPARQL endpoint

Hello. In relation to a project I’m working on, I’m trying to get the oldest date that’s related to a resource in Wikipedia. I used the following query in DBpedia’s public SPARQL explorer to get the first 100 birth dates (although I’m interested in any object of type xsd:date):

select ?object ?date ?link
where {
 ?object dbo:birthDate ?date .
 ?object foaf:isPrimaryTopicOf ?link .
order by ?date
limit 100

However, dates seem to sorted alphabetically instead of numerically. Is this the expected behavior for Openlink Virtuoso? I tried sorting dates of a small dataset with Apache Jena and it works as expected.