OK, so I've looked at this, and the short answer is that the MementoService (which is some rather old code that is used internally by ViewModel) was never updated to work with java.util.Date.
Looking at the code, though, it seems to just do a toString() of the object to be added to the memento. For the data types that are supported, this is unambiguous, but I don't think it would be for a java.util.Date.
Rather than fix/enhance, my suggestion is just to use @XmlRootElement instead. This will bring in the whole power of JAXB for serialization, so should be much more robust.