[Python-Dev] Status on PEP-431 Timezones

[Stuart Bishop]

On 14 April 2015 at 21:04, Lennart Regebro <regebro at gmail.com> wrote:
> OK, so I realized another thing today, and that is that arithmetic
> doesn't necessarily round trip.
>
> For example, 2002-10-27 01:00 US/Eastern comes both in DST and STD.
>
> But 2002-10-27 01:00 US/Eastern STD minus two days is 2002-10-25 01:00
> US/Eastern DST
> However, 2002-10-25 01:00 US/Eastern DST plus two days is 2002-10-27
> 01:00 US/Eastern, but it is ambiguous if you want DST or not DST.
> And you can't pass in a is_dst flag to __add__, so the arithmatic must
> just pick one, and the sensible one is to keep to the same DST.

>>> import pytz
>>> from datetime import datetime, timedelta
>>> tz = pytz.timezone('US/Eastern')
>>> dt = tz.localize(datetime(2002, 10, 27, 1, 0), is_dst=False)
>>> dt2 = tz.normalize(dt - timedelta(days=2) + timedelta(days=2))
>>> dt == dt2
True
>>>
>>> tz.normalize(dt - timedelta(days=2))
datetime.datetime(2002, 10, 25, 2, 0, tzinfo=<DstTzInfo 'US/Eastern'
EDT-1 day, 20:00:00 DST>)
>>> tz.normalize(tz.normalize(dt - timedelta(days=2)) + timedelta(days=2))
datetime.datetime(2002, 10, 27, 1, 0, tzinfo=<DstTzInfo 'US/Eastern'
EST-1 day, 19:00:00 STD>)


2002-10-27 01:00 US/Eastern is_dst=0 is after the DST transition
(EST). Subtracting 48 hours from it crosses the DST boundary and
should give you 2002-10-27 02:00 US/Eastern is_dst=1, prior to the DST
transition (EDT). Adding 48 hours again goes past 2002-10-27 01:00
EDT, crosses the DST boundary, and gives you back 2002-10-27 01:00
EST.


-- 
Stuart Bishop <stuart at stuartbishop.net>
http://www.stuartbishop.net/