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Re: [ontolog-forum] Time representation

To: "[ontolog-forum] " <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Pat Hayes <phayes@xxxxxxx>
Date: Mon, 21 Jan 2008 10:18:26 -0600
Message-id: <p06230908c3ba6a5b4d58@[]>
At 9:37 AM -0500 1/21/08, John F. Sowa wrote:

 > what is the accepted syntax for pTim
 > can pTim be a timestamp?  is there a prescribed format
 > for time/date in ikl

The dyadic relation PTim (point in time) is not an official
part of either conceptual graphs or IKL.  It is part of an
ontology for time, which I commonly use with CGs.  (And yes,
I would be happy to send or point to the ontology, if I had
it written down in a convenient form.  Right now, it is
scattered in a very inconvenient form.)

There are a lot of ontologies for time, in various notations, all pretty similar except of course for the all-important details. I wrote a survey of some of the many options some time ago (all written in KIF), which is available here
and since then a fairly 'standard' ontology was put together for the W3C OWL-based SWServices project by Jerry Hobbs and Feng Pan, originally written in a conventional FOL notation and then as far as possible transcribed into OWL.
There is a lot of commentary, advice, etc.., about this, see for example http://www.geospatialsemanticweb.com/2006/09/29/tips-on-using-the-w3c-owl-time-ontology
and it contains a lot of new stuff, eg a complete ontology of international time-zones and so on. It is probably the most thoroughly worked out 'practical' time ontology.
BTW, if you see a reference to DAML-Time, it was the early version of this, now obsoleted.

The XMLSchema datatype scheme has a fairly complex temporal naming system worked out for dates and times, but it gets rather tangled when it tries to merge months and days.

As I use it, the first argument of PTim is some physical event
or situation, and the second argument is some time specification.
Since I assume that time "points" can never be observed or
specified as true points, there is always an implicit granularity.

Pity you called them 'points', then, as they are in fact intervals :-) How do you handle endpoints of intervals: are they intervals also? One problem with this approach is that the Allen relationships change when you change granularity levels, which wrecks some reasoners.

Therefore, the second argument could be any of the following,
each of which has a default granularity determined by the least
significant digit of the specification:

    [Date: "21 January 2008"] -- granularity of 1 day

    [Time: "21 January 2008, 14:03"] -- granularity, 1 minute

    [Time: "21 January 2008, 14:03:16.217"] -- granularity, 1 ms

Cute. The OWL-Time used a similar device, borrowed I believe from the CYC temporal ontology, in which each successive layer of granularity is treated as a 'selection' function on the previous level, so that 21 January 2008 has the structure 'the 21st day of( the first month of( the 2008th year of(The Christian Calender)))'. This makes both temporal and logical sense.

As pure logics, CGs, CL, and IKL have no ontology or prescribed
format for time, space, or anything else.  Any string enclosed
in double quotes is interpreted as a name in those languages.
Therefore, "21 January 2008, 14:03:16.217" would be the name
of some entity of type Time.  The assumptions about granularity
are not official parts of CGs or any of the other languages.

But any project that used any of those languages would have to
adopt some standard representation for times and dates, and
the ontology would also have to include axioms that stated any
default assumptions about granularity.

Do you have any granularity axioms? That is one of the hardest ontological problems, in my experience.



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