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Re: [ontolog-forum] Time representation

To: "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Ed Barkmeyer <edbark@xxxxxxxx>
Date: Wed, 23 Jan 2008 13:25:44 -0500
Message-id: <479786A8.90703@xxxxxxxx>
Carl Reed wrote:    (01)

> If the forum is going to begin discussing how to express time and the
> semantics of time, I would seriously suggest the group look at the various
> ISO standards and related IETF standards that dal with expressing time. If
> you are interested in expressing time intervals etc, I would suggest you
> look at the OGC Observations and Measurements standard.    (02)

Well, we need to catalog the concerns:    (03)

ISO 1000 deals with time as a measurement, and there are reference 
publications of ISO and NIST on the subject of stating the qualities of 
a measurement and characterizing uncertainty.  And there are specific 
sections on time measurements, which relate to the nature of the 
referents and the nature of the measurement technologies.    (04)

ISO 8601 and several IETF standards deal with standard representations 
of durations and "points in time", but they are thin on the semantics of 
the individual values -- they have no "model" of time.  In particular, 
they deal with precision in representation of a time value, but not 
accuracy.  In general, they see a time "value" as a term drawn from a 
standard vocabulary that is used as a label on an information 
unit/record. They make no clear assumptions about how that vocabulary is 
used or interpreted.    (05)

The CCTS/EDIFACT treatment of date/time is similar to the above, but 
they add standard labels for common usages as well.  But they still 
don't specify how the time value is interpreted.  They assume, in fact, 
that the interpretation is based on law or business agreement that is 
applicable to the information exchange in which the value is used.    (06)

There are other IETF standards, and telecommunications industry (ITU-T) 
standards, that deal with issues of timestamping and clock 
synchronization for networked activities, and there is more work on that 
in the Distributed (systems) Management Task Force (dmtf.org).  These 
specifications have a model of time and accuracy that is used to define 
temporal relationships among "real-time" events, and those relationships 
in turn are used in standard rules of practice.    (07)

There is the "engineering" treatment of time in ISO 15926 and ISO 
11876(?), as someone else indicated.  These things have a model of time 
as a space/time dimension (a) in the definition of a physical object, a 
behavior or a process, or (b) in specifying "effectivity" of 
observations, predictions and decisions.  "effectivity" = when, where, 
and for what, an observation is valid or a decision is applicable. 
These two uses are nearly different concepts: the former, and some 
instances of the latter, are scientific measurements; most instances of 
the latter are industrial convention.  The problem is that in processes 
involving materials that evolve, there are decisions taken at points in 
the process that are named by relative time, but based on the predicted 
properties of the material, but the general model of "effectivity" also 
applies to the decision to configure the processing equipment for a run 
three days after another business event.    (08)

There is a CCSDS standard for representation, reference, and accuracy of 
time in astronomical observations.  The intent of this specification is 
to label observations and allow observations made in different parts of 
the solar system (terrestrial, satellite, vehicle) to be correlated by 
time (among other space/time dimensions).    (09)

I would appreciate a reference to the OGC specification, and a quick 
precis of the nature of its treatment of "time".    (010)

My point is only that "time" has been a subject of standards work for 50 
years.  Each of these standards has a particular purpose, and deals with 
particular aspects of the concept "time" that are relevant to that 
purpose.  So you need to decide what aspects you care about in order for 
any of them to be relevant (or even comprehensible, which was Pat 
Hayes's point).    (011)

Bear in mind also that standards are about establishing conventions for 
representation and conventions for particular usages, with the goal of 
establishing interoperability of people, publications, equipment and 
software in specific endeavours.  It is typical that they are devoid of 
semantics beyond that required for the particular interoperability goal.    (012)

-Ed    (013)

Edward J. Barkmeyer                        Email: edbark@xxxxxxxx
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8263                Tel: +1 301-975-3528
Gaithersburg, MD 20899-8263                FAX: +1 301-975-4694    (014)

"The opinions expressed above do not reflect consensus of NIST,
  and have not been reviewed by any Government authority."    (015)

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