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RE: date in user's timezone

From: <Noah_Mendelsohn@lotus.com>
Date: Fri, 27 Oct 2000 16:09:28 -0400
To: "Graham Ross" <gar@thinkshare.com>
Cc: petsa@us.ibm.com, wca@thinkshare.com, www-xml-schema-comments@w3.org
Message-ID: <OF1E8978BE.1B131FD4-ON85256985.006D7693@lotus.com>
The point you may be missing is that, for better or worse, the time zone 
is not included in the value space of the time type (right Paul & Ashok? 
it's not in 8601 section 5.4 and its not in Datatypes section 3.2.6, both 
of which are referenced in defining value spaces for date-related types). 
Think about it this way: 10.0 and 1.0E+1 are the _same_ float, represented 
differently.  Nothing in the value space allows you to distinguish one 
from the other.  In that respect, the lexical representations are 
primarily a convenience, although we could certainly burn a lot of energy 
debating the degree to which applications should attempt to derive useful 
information from the differences between these two.

In the same spirit, the time 1999-05-31T13:20:00.000-05:00 and 
1999-05-31T18:20:00.000 _are_ the same time.  The datatype signals no 
semantic differences between the two, just as with the floating point 
example above.  The number (-5) does not appear in the value space for the 
first form.

Now, I say for better or worse because one can certainly imagine 
situations in which you would like to do computation on the time zone.  It 
is also the case that dealing with time zones at all is known to be quite 
subtle business (Our company has actually had bugs reported in our 
calendar system such as: users scheduled appointments far in advance, and 
legislatures enacted new timezone laws between the scheduling of the 
appointment and the appointment itself.  The alarm rings an hour late.) 
When scheduling is done across political and geographic boundaries, this 
all gets very difficult.  My own preference would have been to leave the 
time zone specification out of the schema datatype entirely, and require 
that all times be explicitly UTC, because it would be clearer that the 
timezone has no semantic significance.  Nonetheless, it is obviously a 
convenience to be able to use the +/- zone notation in systems where time 
zones are stable and users wish to encode local times.  

Noah Mendelsohn                                    Voice: 1-617-693-4036
Lotus Development Corp.                            Fax: 1-617-693-8676
One Rogers Street
Cambridge, MA 02142
Received on Friday, 27 October 2000 16:15:18 UTC

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