This line in schemaorg.owl version 15.0 should use http://www.w3.org/2001/XMLSchema#date. dat is not one of the XML Schema datatypes (primitive or derived). A grep shows this typo seems to have been present since release 3.9.

This can cause a runtime error in tools that consider certain namespaces to have a closed membership set, such as the XML Schema datatypes (i.e. the tool does typo checks for standard IRIs).

Can this typo be corrected for the next release?