[Bug 23473] dateTime equality questions

https://www.w3.org/Bugs/Public/show_bug.cgi?id=23473

--- Comment #1 from C. M. Sperberg-McQueen <cmsmcq@blackmesatech.com> ---
I believe that you are right that the three examples you cite are equal, for
the reason you give.  They denote the same moment in the timeline of the earth.

They are not, however, identical in XSD 1.1.  (They were identical in XSD 1.0,
but the working groups responsible for XSLT and XQuery refused to accept that
decision and defined the time zone as being an intrinsic part of the value, in
order to distinguish cases like these from each other.  To align with the XSLT
and XQuery specifications, XSD 1.1 changed its definition of the value spaces
for the relevant types.)

By "extreme" time zone offsets, the note means time zone offsets near the
minimum of maximum allowed values.  (Specifically, I believe the phenomena the
note is trying to refer to appear with time zone offsets whose difference from
UTC is 720 minutes or greater.)  I believe the concept of equality it is
talking about is the one defined elsewhere (that is:  I do not think "equal" is
a typographic error for "identical", nor do I think that some other notion is
involved).

The note is indeed a bit puzzling.  The record shows it was part of a change
proposal adopted by the XML Schema WG on 11 February 2005 [1]; the minutes [2]
do not record any particular discussion of the note.  I suspect that when the
note was originally drafted, the drafter was thinking of all of the date/time
datatypes together, and not specifically of xs:dateTime; for the type xs:time,
for example, the value xs:time('23:59:00-14:00') is not (as far as I can see at
the moment) equal to any other xs:time value. This is a consequence of the way
in which time values are augmented in order to calculate timeOnTimeline values.
 Since dateTime values are not augmented in this way, I do not see any way for
a value of xs:dateTime to fail to be equal to other dateTime values.  (But the
history of the working group's discussions in this area provides a sobering
warning against assuming that there are none, because we can't think of any at
the moment.)

I am inclined to think that the note is misleading to suggest that dateTime
values with time zone offsets exceeding 720 minutes may be unequal to any
values with lower time zone offsets; this is true for some date/time types, but
(it seems to me today) not to dateTime.  In a perfect world, the responsible WG
would issue an editorial correction.  But in view of the fact that the note is
not strictly speaking in error (it doesn't say that some dateTime values are
not equal to any others, only that there may be such), that it's non-normative
(being a note), that the area is extremely error prone and all changes must be
approached with extreme caution, and that the responsible WG has very limited
resources, I expect it's unlikely that this mistake (if I am right that it is
one) will be fixed soon.

[1]
https://www.w3.org/XML/Group/2004/06/xmlschema-2/datatypes.rq122.200502.html
(member-only link) 

[2] https://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2005Feb/0051.html
(member-only link)

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Received on Wednesday, 9 October 2013 19:29:15 UTC