[Python-Dev] Status on PEP-431 Timezones

MRAB python at mrabarnett.plus.com
Mon Jul 27 17:26:28 CEST 2015

On 2015-07-27 15:59, Paul Moore wrote:
> On 27 July 2015 at 14:59, R. David Murray <rdmurray at bitdance.com> wrote:
>> I have a feeling that I'm completely misunderstanding things, since
>> tzinfo is still a bit of a mystery to me.
> You're not the only one :-)
> I think the following statements are true. If they aren't, I'd
> appreciate clarification. I'm going to completely ignore leap seconds
> in the following - I hope that's OK, I don't understand leap seconds
> *at all* and I don't work in any application areas where they are
> relevant (to my knowledge) so I feel that for my situation, ignoring
> them (and being able to) is reasonable.
> Note that I'm not talking about internal representations - this is
> purely about user-visible semantics.
Would it help if it was explicit and we had LocalDateTime and

> 1. "Naive" datetime arithmetic means treating a day as 24 hours, an
> hour as 60 minutes, etc. Basically base-24/60/60 arithmetic.
> 2. If you're only working in a single timezone that's defined as UTC
> or a fixed offset from UTC, naive arithmetic is basically all there
> is.
> 3. Converting between (fixed offset) timezones is a separate issue
> from calculation - but it's nothing more than applying the relevant
> offsets.
> 4. Calculations involving 2 different timezones (fixed-offset ones as
> above) is like any other exercise involving values on different
> scales. Convert both values to a common scale (in this case, a common
> timezone) and do the calculation there. Simple enough.
> 5. The problems all arise *only* with timezones whose UTC offset
> varies depending on the actual time (e.g., timezones that include the
> transition to DST and back).
> Are we OK to this point? This much comprises what I would class as a
> "naive" (i.e. 99% of the population ;-)) understanding of datetimes.
> The stdlib datetime module handles naive datetime values, and
> fixed-offset timezones, fine, as far as I can see. (I'm not sure that
> the original implementation included fixed-offset tzinfo objects, but
> the 3.4 docs say they are there now, so that's fine).
> Looking at the complicated cases, the only ones I'm actually aware of
> in practice are the ones that switch to DST and back, so typically
> have two offsets that differ by an hour, switching between the two at
> some essentially arbitrary points. If there are other more complex
> forms of timezone, I'd like to never need to know about them, please
> ;-)
> The timezones we're talking about here are things like
> "Europe/London", not "GMT" or "BST" (the latter two are fixed-offset).
> There are two independent issues with complex timezones:
> 1. Converting to and from them. That's messy because the conversion to
> UTC needs more information than just the date & time (typically, for
> example, there is a day when 01:45:00 maps to 2 distinct UTC times).
> This is basically the "is_dst" bit that Tim discussed in an earlier
> post. The semantic issue here is that users typically say "01:45" and
> it never occurs to them to even think about *which* 01:45 they mean.
> So recovering that extra information is hard (it's like dealing with
> byte streams where the user didn't provide details of the text
> encoding used). Once we have the extra information, though, doing
> conversions is just a matter of applying a set of rules.
> 2. Arithmetic within a complex timezone. Theoretically, this is simple
> enough (convert to UTC, do the calculation naively, and convert back).
> But in practice, that approach doesn't always match user expectations.
> So you have 2 mutually incompatible semantic options - 1 day after 4pm
> is 3pm the following day, or adding 1 day adds 25 hours - either is a
> viable choice, and either will confuse *some* set of users. This, I
> think, is the one where all the debate is occurring, and the one that
> makes my head explode.
> It seems to me that the problem is that for this latter issue, it's
> the *timedelta* object that's not rich enough. You can't say "add 1
> day, and by 1 day I mean keep the same time tomorrow" as opposed to
> "add 1 day, and by that I mean 24 hours"[1]. In some ways, it's
> actually no different from the issue of adding 1 month to a date
> (which is equally ill-defined, but people "know what they mean" to
> just as great an extent). Python bypasses the latter by not having a
> timedelta for "a month". C (and the time module) bypasses the former
> by limiting all time offsets to numbers of seconds - datetime gave us
> a richer timedelta object and hence has extra problems.
> I don't have any solutions to this final issue. But hopefully the
> above analysis (assuming it's accurate!) helps clarify what the actual
> debate is about, for those bystanders like me who are interested in
> following the discussion. With luck, maybe it also gives the experts
> an alternative perspective from which to think about the problem - who
> knows?
> Paul
> [1] Well, you can, actually - you say that a timedelta of "1 day"
> means "the same time tomorrow" and if you want 24 hours, you say "24
> hours" not "1 day". So timedelta(days=1) != timedelta(hours=24) even
> though they give the same result for every case except arithmetic
> involving complex timezones. Is that what Lennart has been trying to
> say in his posts?

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