I was wondering if anyone else has worked with "ancient" dates in ISO 8601/W3CDTF. We are working with museums who have artifacts from before the year 0 (both eg. 800 BCE and fossils), however ISO 8601 seems to be suited best for dates within the Gregorian calendar.
I see that the latest version ISO 8601:2000  (sections 18.104.22.168, 4.7 and 22.214.171.124) does allow extensions so it is possible to have "negative" years and also to add extra digits to the year (ie. going over 9999 years), though it does state this use "should only be done by agreement of the partners in information interchange."
The W3C's Date and Time Formats (W3CDTF) Note  discusses 6 levels of granularity for ISO 8601 dates plus formats for presentation of these. I'm guessing adding more year digits and/or a preceeding minus sign (as per ISO 8601:2000) would not comply with this note.
Therefore, does anyone have suggestions/experience in encoding old dates?
It seems to me the options are:
1. Encode in ISO8601:2000, unsure what encoding scheme to state - DC doesn't include ISO8601, perhaps submit ISO8601 as a scheme to DC? Though there is still the issue that "-YYYYYY" is an "extension" rather than "standard" use within ISO 8601.
2. Encode in ISO8601:2000 and request W3C to update their "Date and Time Formats" note to reflect the newer version, in particular to handle BCE dates (so I can then state the encoding scheme as "W3CDTF")
3. Encode in another date standard (I haven't located one for old dates yet)
National Library of New Zealand