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Re: ISSUE-126 (Revisit Datatypes): The list of normative datatypes should be revisited

From: Alan Ruttenberg <alanruttenberg@gmail.com>
Date: Thu, 19 Jun 2008 12:26:06 -0400
Cc: "'OWL Working Group WG'" <public-owl-wg@w3.org>
Message-Id: <78DAA9C4-D0DD-4B1F-864C-323BC62D6F7E@gmail.com>
To: Evan Wallace <ewallace@cme.nist.gov>

I was looking into xsd date types, as (all things being equal) I would  
think we would want to support them. One of the things that surprised  
me was that dates are not a total order. http://www.w3.org/TR/xmlschema-2/#dateTime-order

Basically, if one of the dates to be compared has a time zone and the  
other does not, and the dates without considering the timezone are  
less than 14 hours apart, then the dates are incomparable wrt order.

I'm not sure how much this complicates implementations, but note that  
it would mean that the union of date > x and complementof(date >x) is  
not all dates.

If this poses a problem, and we think date ranges are important,  we  
may need to specialize the xsd dates (and related date types, such as  
gYearMonth) into those which require a timezone, and those that  
require there not be one, and then only offer facets on these subtypes.
The xsd spec alludes to such a strategy http://www.w3.org/TR/xmlschema-2/#totally-ordered-instants 
  though still says that the min and max facets are available for  
dateTime, despite the incomparables.

I'm also not sure what complications leap year calculations bring in  
to the picture. Following Boris' example, suppose you have gMonthDay  
is Feb 29 and x < gYear < y, then this is only satisfiable for some  
values of x and y and you need to do the leap year calculation to be  
able to tell. In this case timezone doesn't matter, but what about Feb  
28 6PM 20xx < dateTime <  March 1 6AM 20yy.  Now, depending on whether  
Feb 29, x, and y have timezone specified, and the specific years, you  
might have a situation where you just can't say if it is satisfiable,  
as the facet values can't be compared.

What strategy are you using for handling such oddities in your date  
and time vocabulary? How important is the ability to make date range  
data ranges?

-Alan


On Jun 19, 2008, at 11:47 AM, Evan Wallace wrote:

> The OWL 1 rendering of OWL Time makes use of xsd types for a number  
> of things
> including identifying a year and month for a time interval  
> associated with a
> DateTimeDescription.  It also carefully uses integer for datatype  
> properties using
> ordinal numbers to identify say a day of the week or a week in a  
> WeekYear calendar,
> while still using xsd decimal for seconds in a date-time.  While it  
> would probably be
> a good thing to change OWL Time, in any case, to make use of new  
> constructs in OWL 2,
> we really need to understand what datatypes to use for these things  
> if support for integer
> and xsd date types is discouraged.
>
> -Evan
>
> Evan K. Wallace
> Manufacturing Systems Integration Division
> NIST
>
>
Received on Thursday, 19 June 2008 16:26:57 GMT

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