We are currently following XSD 1.0:
XSD 1.1 changes the meaning of the year 0 and negative years. http://www.w3.org/TR/xmlschema11-2/#dateTime We currently do not follow this standard.
| Status | Subtype | Assigned | Task | ||
|---|---|---|---|---|---|
| Open | None | T87764 Bugs related to time datatype (tracking) | |||
| Duplicate | None | T93256 Documentation of year 0 and negative years |
This is described as documentation. But isn't there some part of Wikidata that parses input and stores it in some unstated format? Isn't there some other part of Wikidata that parses the stored format and output it in what seems to be the canonical output format, JSON?
The documentation on the JSON format seems to be at https://git.wikimedia.org/blob/mediawiki%2Fextensions%2FWikibase/master/docs%2Fjson.wiki
This says the JSON format resembles ISO 8601. Since ISO 8601 considers 0000 to be equal to 1 BC, could someone point out the documentation that says otherwise?
Which edition of ISO 8601?
@JanZerebecki did some research in T94064: Date of +0000-01-01 is allowed but undefined in wikibase but is not allowed in xsd:dateTime as implemented by blazegraph. This ticket probably classifies as a duplicate of T94064 now.
ISO 8601:2004_E states
--calendar year is, unless specified otherwise, represented by four digits. Calendar years are numbered in
ascending order according to the Gregorian calendar by values in the range [0000] to [9999]. Values in
the range [0000] through [1582] shall only be used by mutual agreement of the partners in information
interchange."
That version also indicates negative years are acceptable by mutual agreement, and that 0000 is a leap year in the proleptic Gregorian calendar. If 1 BC were designated -0001, that would be the year that's a leap year, but since 0000 is the leap year, -0001 must correspond to 2 BC.