On 2022-04-17 02:14:44 +1000, Chris Angelico wrote: > On Sun, 17 Apr 2022 at 02:03, Peter J. Holzer <hjp-pyt...@hjp.at> wrote: > > On the contrary. When a datetime is timezone aware, it must use that > > timezone's rules. Adding one day to a datetime just before a DST switch > > must add 23 or 25 hours, not 24. This is NOT naive. > > > > (There is an ambiguity, though: Should 2021-03-27T12:00 CEST - > > 2021-03-26T12:00 CET return 1 day or 25 hours? Both results are correct, > > and depending on context you might prefer one or the other). > > > > That's exactly the problem. A day IS 24 hours *and* it is the time > period required for you to get to the same clock on the following > date. It's fundamentally ambiguous. > > Let's take this out of Python altogether for a moment. Imagine that > it's 9AM Thursday. You say to your friend, hey, let's meet up again, > same time next week. Are you planning to meet 168 hours later, or at > 9AM the following Thursday? > > OF COURSE you mean 168 hours later. That's what "next week" means.
Of course NOT. It means that only in 50 out of 52 weeks. A 4% error rate
is more than enough to make me acutely aware that it isn't true in
general. Also, won't count off 168 hours but look at my calendar/clock.
> OF COURSE you're meeting at 9AM the following Thursday. That's what
> "next week" means.
This. Certainly when meeting a friend. Almost certainly when dealing
with humans (if they are in a different time zone we may have to agree
on a time zone for "same time"). For a technical process I *might*
simplify "1 week" to "168 hours", but only if the spec gives me
sufficient leeway.
> And they can't both be true if DST is changing.
>
> So which one is it? Which one do you get when you add days=7 to a datetime?
For adding a datetime and timedelta I think the answer is clear.
But subtracting two datetimes is ambiguous.
hp
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| | | h...@hjp.at | -- Charles Stross, "Creative writing
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