[Python-Dev] Status on PEP-431 Timezones

[Akira Li]

Alexander Belopolsky <alexander.belopolsky at gmail.com> writes:

> On Wed, Apr 8, 2015 at 3:57 PM, Isaac Schwabacher <ischwabacher at wisc.edu>
> wrote:
>>
>> On 15-04-08, Alexander Belopolsky wrote:
>> > With datetime, we also have a problem that POSIX APIs don't have to
> deal with: local time
>> > arithmetics. What is t + timedelta(1) when t falls on the day before
> DST change? How would
>> > you set the isdst flag in the result?
>>
>> It's whatever time comes 60*60*24 seconds after t in the same time zone,
> because the timedelta class isn't expressive enough to represent anything
> but absolute time differences (nor should it be, IMO).
>
> This is not what most uses expect.  The expect
>
> datetime(y, m, d, 12, tzinfo=New_York) + timedelta(1)
>
> to be
>
> datetime(y, m, d+1, 12, tzinfo=New_York)

It is incorrect. If you want d+1 for +timedelta(1); use a **naive**
datetime. Otherwise +timedelta(1) is +24h:

  tomorrow = tz.localize(aware_dt.replace(tzinfo=None) + timedelta(1), is_dst=None)
  dt_plus24h = tz.normalize(aware_dt + timedelta(1)) # +24h

*tomorrow* and *aware_dt* have the *same* time but it is unknown how
 many hours have passed if the utc offset has changed in between.
*dt_plus24h* may have a different time but there are exactly 24 hours
 have passed between *dt_plush24* and *aware_dt*
http://stackoverflow.com/questions/441147/how-can-i-subtract-a-day-from-a-python-date