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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