[Python-Dev] Status on PEP-431 Timezones

[Sven R. Kunze]

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
> <ronaldoussoren at 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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