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>> For example, I think that "What is 2002-09-28 plus a year and a
>> day?" is also a simple question with a well-defined answer; I don't
>> think it should be an error to try to ask it.
> Ah, but what is 2004-02-29 plus a year and a day?
> Time labeling is an impossible compromise between incommensurable
> astronomical cycles, and can never be really clean.
Sure, that means sometimes it has to be a bit arbitrary, but it
doesn't mean that it's impossible to create algorithms that give
consistent (if not absolutely "correct") results.
The algorithm in the W3C XML Schema Datatypes Recommendation adds the
year/month component first (which would give 2005-02-29), then "pins"
the result of that to give a legal date (i.e. 2005-02-28), and then
adds the day and time components of the duration (to give the final
answer of 2005-03-01).