I agree and my 2 cents: I can expect something different depending on
the timezone and DST if I add
years
months
weeks
days
hours
minutes
seconds
to a given datetime
Even though, in 90% of the cases, there is a more or less obvious
conversion formula between all of them. But consider months to days.
That is not clear at all.
On 27.07.2015 19:11, Chris Angelico wrote:
On Tue, Jul 28, 2015 at 12:57 AM, Ronald Oussoren
<ronaldousso...@mac.com> wrote:
IMHO “+ 1 days” and “+ 24 hours” are two different things. Date
arithmetic is full of messy things like that. “+ 1 month” is another
example of that (which the datetime module punts completely
and can be a source of endless bikeshidding).
https://www.youtube.com/watch?v=ppfpa5XgZHI
MATLAB defines "+ 1 month" as, if I'm not mistaken, "add the time it
would take to go from the beginning of time to the beginning of
January of the year 0 (which is totally a thing, by the way)". I'm
fairly sure that this is the most WAT-worthy definition possible, as
it means that adding one month does nothing, and adding two months
adds the length of January (31 days)... and adding three months adds
January + February, *in a leap year*.
But I agree that adding days and adding hours are different things. If
I add one day, I expect that the time portion should not change, in
the given timezone. (With the exception that DST switches might mean
that that time doesn't exist.) If I add 86400 seconds, I expect that
it should add 86400 ISO seconds to the time period, which might not be
the same thing. If you convert a datetime to a different timezone, add
86400 seconds, and convert back to the original timezone, I would
expect the result to be the same as adding 86400 seconds to the
original, unless there's something seriously bizarre going on with the
size of the second. But if you convert, add 1 day, and convert back,
you will get a different result if the two differ on DST. Does that
sound plausible?
ChrisA
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