WDQS holds dates as xsd:dateTime using the proleptic Gregorian calendar. - https://www.mediawiki.org/wiki/Wikibase/Indexing/RDF_Dump_Format#Time
Julian calendar dates entered into Wikidata are stored as such in JSON.
They are converted to proleptic Gregorian calendar dates for WDQS
However, WDQS stores the original - Julian - calendar as the value for wikibase:timeCalendarModel
It follows that a Julian date entered and displayed as expected in Wikidata - such as https://www.wikidata.org/wiki/Q16931292#P571 - 22 June 1498 - is in WDQS represented by the following value set:
By any reasonable definition, this is an error. WDQS is representing the value as 1 July 1498 Julian, when, at best, it should be 1 July 1498 Gregorian, and ideally should be 22 June 1498 Julian.
I think this date handling needs a rethink, perhaps along the line of:
given: Julian date in wikidata
when: WDQS reports on the date
- ps:Pnnn value should be the Julian date - 22 June 1498
- psv:Pnnn/wikibase:timeValue should be the Julian date - 22 June 1498
- psv:Pnnn/wikibase:timeCalendarModel) should be wd:Q1985786 (proleptic Julian calendar)
- psn:Pnnn/wikibase:timeValue should be the Gregorian date - 1 July 1498
- psn:Pnnn/wikibase:timeCalendarModel) should be wd:Q1985727 (proleptic Gregorian calendar)