Depending on what platform is going to be used to process the RDF generated by RdfBuilder, we'd want XSD 1.1 dates (using astronomical year numbering) or XSD 1.0 (using traditional year numbering).
In order to implement this switch, we have to ce certain about which numbering the internal erepresentation of Gregorian and Julian dates uses. The current assumption is that these use traditional numbering (-0001 means 1 BCE, 0000 is invalid).