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DATETIME  November 2009

DATETIME November 2009

Subject:

Re: UTC [SEC=UNCLASSIFIED]

From:

"J. Prevost" <[log in to unmask]>

Reply-To:

Discussion of the Developing Date/Time Standards <[log in to unmask]>

Date:

Mon, 30 Nov 2009 20:41:19 -0500

Content-Type:

text/plain

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Parts/Attachments

text/plain (71 lines)

On Mon, Nov 30, 2009 at 19:25, David Bromage <[log in to unmask]> wrote:
{... various ponderings on historical point of time zones ...}
> Taking as an example a well known date and time prior to this, letís say
> 1776-07-04T11:00:00, does a time zone actually add any meaning?

Considering that time zones are optional, is there even a problem
here?  A date-time expressed without a time zone is a date-time with
no time zone, which is distinct from a date-time with the UTC time
zone.

So it is easy enough to simply not include time zones where time zones
are completely inappropriate.

Note, however, that it can be difficult to compare or take the
difference between date-times without a time zone.  The XMLS datatypes
spec specifies that if you have two datetimes without timezones, you
should compare them directly, but if you have a mix of one with a
timezone and one without, the result of comparison may be
indeterminate (http://www.w3.org/TR/xmlschema-2/#dateTime-order).

A more apt question *might* be whether there is some need for
supporting historical time zones (for example, by reference to UT,
GMT, or location for local time references) within the standard.
Unless historical time reference point support is of great value, I
would suggest that it would overcomplicate matters greatly to attempt
to specify them to the point where there's a standard set of rules for
comparing datetimes with different reference points.

If there's no or very limited desire for historical time reference
points, I would suggest that the spec merely mentions "datetimes
before the definition of UTC in 1972 SHOULD NOT include time zone
information".

If there is sufficient desire for historical time reference points, I
would suggest extending the specification to allow time reference
specifications such as "@Rome" to be given in place of time zones for
historical dates.  These could be purely informational, with exact
definitions of comparison between datetimes with different reference
points being undefined, and the suggestion that "historical datetimes
SHOULD be compared as if they have no time zone information at all,
even if a time reference point is specified."

If there is a very large desire for historical time reference points
with the ability to meaningfully compare values with different
reference points, I'm not sure what to say.  That's a very hard
problem.


This entire time reference problem is of course related to another
problem which might be more important: what is the right way to work
with dates from distinctly different calendars?  When working with
historical source material where dates are actually specified in the
Julian calendar, it seems like it would be inappropriate (or at least
not always appropriate) to convert the values to semantically equal
dates in the Gregorian calendar for storage.  Nevertheless, these
dates might sometimes need to be stored (should they be validated?
What about really obscure calendars?) in a common location and format.

This is a similar problem to the time zone problem, because it relates
to the *interpretation* of the values, including the range of valid
values, how values within one "scope of interpretation" should be
compared, and particularly troublesomely, how values in different
scopes should be compared.  Datetimes within a single calendar are
pretty close to directly comparable textually, particularly if you
ignore everything below day resolution (although this is not
completely correct).  Datetimes within different calendar systems must
be interpreted in order to compare them.


John.

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