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


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