Rhodri James wrote:
On Fri, 09 Oct 2009 13:39:43 +0100, Tim Chase
<python.l...@tim.thechases.com> wrote:
Month arithmetic is a bit of a mess, since it's not clear how
to map e.g. Jan 31 + one month.
"Jan 31 + one month" usually means "add one to the month value and
then keep backing off the day if you get an exception making the
date", so you'd get Feb 31, exception, Feb 30, exception, Feb 29,
possibly an exception, and possibly/finally Feb 28th. This makes
pretty intuitive sense to most folks and is usually what's meant.
I've found that issues and confusion stem more from the
non-commutative reality that "Jan 31 + (1 month) + (-1 month) != Jan
31 + (-1 month) + (1 month)" or the non-associative "Jan 31 + (1 month
+ 1 month) != (Jan 31 + 1 month) + 1 month" :-/
I'd hazard a guess that what we're actually seeing is people mentally
rebasing their indices, i.e. counting from the end of the month rather
than the start, which makes "the last day of January" and "January 31"
not the same thing really. Unfortunately we're very fuzzy about when
we do things like this, which makes it hard on a poor programmer.
And when someone says "January 30", do they really mean the day before
the last day of the month? Where would it end? :-)
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