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RE: required timezone xsd dateTime datatype (ISSUE-138)

From: Boris Motik <boris.motik@comlab.ox.ac.uk>
Date: Tue, 30 Sep 2008 15:35:15 +0100
To: "'Peter F. Patel-Schneider'" <pfps@research.bell-labs.com>, <public-owl-wg@w3.org>
Message-ID: <9B57E0103F0745DE902678867FADEE95@wolf>

Hello,

Although this is certainly good, it does not solve all of the problems regarding the naming of owl:dateTime. In particular, the
identity of owl:dateTime values is different from the identity of xsd:dateTime values. Consider the following two dates that
represent the same time instant but have different time zones:

(1) 2008-10-01T08:00:00+01:00
(2) 2008-10-01T07:00:00+00:00

In owl:dateTime, these two instants are identical (this was decided at the last F2F), which is not the case in the XML Schema
Datatypes 1.1 version of xsd:dateTime (according to http://www.w3.org/TR/xmlschema11-2/#dateTime).

This has important consequences on the semantics of OWL 2. The following ontology is consistent: it uses owl:dateTime so the two
instants are identical, and the functionality restriction is not violated.

(2) FunctionalProperty( a:time )
(3) PropertyAssertion( a:time a:i "2008-10-01T08:00:00+01:00"^^owl:dateTime )
(4) PropertyAssertion( a:time a:i "2008-10-01T07:00:00+00:00"^^owl:dateTime )

In contrast, the following ontology is inconsistent: it uses xsd:dateTime so the two instants are distinct, which causes a violation
of the functionality restriction: 

(5) FunctionalProperty( a:time )
(6) PropertyAssertion( a:time a:i "2008-10-01T08:00:00+01:00"^^xsd:dateTime )
(7) PropertyAssertion( a:time a:i "2008-10-01T07:00:00+00:00"^^xsd:dateTime )

Essentially, xsd:dateTime has a separate time line for each time zone. Because of that, I believe we should not call our datatype
xsd:dateTime even if the XML Schema WG introduces the change Peter mentioned.


In my opinion, if we were to use xsd:dateTime, then we should revise the decision from the last F2F and define the datatype such
that (1) and (2) are different instants. If we were to do that, I don't see then why we should focus only on dates with a time zone:
the XML Schema 1.1 specification provides a clear model for timestamps without time zones and we could just use it.

Regards,

	Boris


> -----Original Message-----
> From: public-owl-wg-request@w3.org [mailto:public-owl-wg-request@w3.org] On Behalf Of Peter F. Patel-
> Schneider
> Sent: 30 September 2008 15:15
> To: public-owl-wg@w3.org
> Subject: required timezone xsd dateTime datatype (ISSUE-138)
> 
> 
> Discussion in the XML Schema WG on 29 September 2008 chronicled at
> http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2008Sep/0025.html
> indicates that they will indeed have a datatype that is xsd:dateTime
> with a required timezone.  They will also have a facet for xsd:dateTime
> that indicates whether the timezone is required, forbidden, or optional.
> 
> The net result is that we can eventually use an xsd name for
> owl:dateTime.  However, I suggest that this does not need to be
> reflected in the current round of publication.
> 
> peter
> 
Received on Tuesday, 30 September 2008 14:37:04 UTC

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