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On Sat, 2003-10-11 at 12:56, Liam Quin wrote:
> Rick mentioned dates and durations, so I'll say a bit more
> about them to show how tricky they are... yes, if you
> subtract 20th June, 1837 from 20th July 1837, yuo get either
> "a month" or you get 30 days.
no - you get 30 days. implicit in specifying the day of the month. it's
only a coincidence that they're the same day. this is part of the point
i'm trying to make. subtract dates and you get days.
now july 1837 - june 1837 is one month. you're subtracting months -
these aren't dates.
> If you then add the resulting duration to 30th January, 2003,
> you get either 30th February 2003 (probably an error) or you
> get nd March 2003, which might or might not be what you wanted.
now there's no error. 30th january + 30 days may or may not be the 29th
february (might be 1st march); january 1837 + 1 (month) is february
30th january + 1 (month) is not defined; but 20th january + 1 (month) is
= 20th february. but then we could say + 1month = last day of month if
the new month doesn't have enough days. equally for - 1 (month)
so with domains we have to be able to be specific - it behaves like a
group; it is defined by these rules; by observation for this user
exceptions are handled this way... etc