[Python-Dev] Status on PEP-431 Timezones

Alexander Belopolsky alexander.belopolsky at gmail.com
Mon Jul 27 17:37:31 CEST 2015

On Mon, Jul 27, 2015 at 10:59 AM, Paul Moore <p.f.moore at gmail.com> wrote:

> 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.

It is slightly more complicated than that.  There are locations (even in
the US, I've heard)
where clocks have been moved back for reasons other than DST.  I described
one such
example in my earlier post [1].  On July 1, 1990, at 2AM, the people of
Ukraine celebrated
their newly acquired independence by moving their clocks  back to 1AM thus
through "1990-07-01T01:45" local time twice.  This happened in the middle
of summer and
daylight savings time was in effect before and after the transition, so you
cannot use isdst
to disambiguate between the first and the second "01:45".

On the other hand, these rare events are not that different from more or
less regular DST
transitions.  You still have either a non-existent or ambiguous local times
interval and
you can resolve the ambiguity by adding 1 bit of information.  The only
question is what
should we call the flag that will supply that information?  IMO, "isdst" is
a wrong name
for dealing with the event I described above.

[1]: https://mail.python.org/pipermail/python-dev/2015-April/139099.html
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