midnight in ISO 8601 (was: Re: Notes on the say-as note)

In http://lists.w3.org/Archives/Public/www-voice/2005AprJun/0035.html,
Dave Pawson wrote:

  00-23 for hours.
  I'd rather stay with iso8601.

Similarly, various other posters to the thread have 
seemed to accept as a premise that ISO 8601 does not
allow "24" in an hours element.

In the interests of having a clear record, perhaps 
it should be pointed out that if one would rather 
"stay with" ISO 8601, then the value 24 really 
does need to be allowed to appear in the hours field 
of time expressions.  The copies of ISO 8601 I have 
on my shelf (the IS of 1988 and a "final draft" 
from 2000) both specifically mention that midnight 
may be denoted either as "00:00:00" or as "24:00:00" 
(clause 5.3.2 in each case).  

I suspect Mr. Pawson was misled by the restriction 
to the range 00-23 in the profile at
http://www.w3.org/TR/NOTE-datetime -- this is a 
change vis-a-vis ISO 8601, not a reflection of a
restriction made by ISO 8601.

In the context of the SSML 1.0 say-as element, it 
seems clear to me that Eira Monstad is right to suggest
that a restriction to the range 0-23 is unhelpful
in the task of describing time expressions in 
unconstrained natural-language text, and that the
range 0-24 would be more useful.


-C. M. Sperberg-McQueen
 World Wide Web Consortium