On 01.07.2015 18:14, Peter F. Patel-Schneider wrote:
On 07/01/2015 07:00 AM, Pierpaolo Bernardi wrote:
On Wed, Jul 1, 2015 at 8:17 AM, Markus Krötzsch markus@semantic-mediawiki.org wrote:
Dear Pierpaolo,
This thread was only about Julian and Gregorian calendar dates. If and how other calendar models should be supported in some future is another (potentially big) discussion. As you said, there are many issues there. Let's first make sure that we handle the "easy" 99.9% of cases correctly before discussing any more complicated options.
Lydia Pintscher in the starting email explained that there's a model for calendars, and unfortunately this model could be (and has been) interpreted in two ways (AFAIU).
My intention was to point out that one of the two interpretations is not sound. This leaves the other one as the only viable one.
Cheers P.
It appears (from the email only---there are no pointers to enduring documentation on the solution that are attached to the relevant classes or poperties) that the chosen method is to store dates in both the source calendar and the proleptic Gegorian calendar (https://www.wikidata.org/wiki/Wikidata:Project_chat#calendar_model_screwup). As you point out, this is not a viable solution for calendars whose days do not start at the same time as days in the proleptic Gegorian calendar (unless, of course, there is time and location information also available).
The Wikidata date implementation intentionally restricts to dates that are compatible with the Gregorian calendar. Although the system refers to Wikidata item ids of calendar models to denote "Proleptic Gregorian" and "Proleptic Julian", the system does not allow users or bots to enter arbitrary items as calendar model.
My understanding (and the implementation in WDTK) is that all dates are provided in Gregorian calendar with a calendar model that specifies how they should be displayed (if possible). The date in the source calendar is for convenience and maybe for technical reasons on the side of the PHP implementation. At no time should the source calendar date be impossible to convert to Gregorian. We have had extensive discussions about this point -- Gregorian must remain the main format at all times.
This does not mean that we cannot have more models in the future. There is (currently unused) timezone information, which can be used to store offsets. Once fully implemented, this might allow exact conversion from calendar models that have another start for their days. So maybe this is not a case of real incompatibility. However, the timezone support for current dates needs to be finished before discussing the next steps into more exotic calendars.
Best regards,
Markus