[Python-Dev] Status on PEP-431 Timezones (original) (raw)

Lennart Regebro regebro at gmail.com
Wed Apr 15 10:09:05 CEST 2015

Yeah, I just realized this. As long as you use timedelta, the difference is of course not one day, but 24 hours. That solves the problem, but it is surprising in other ways.

In US/Eastern datetime.datetime(2002, 10, 27, 1, 0) - datetime.timedelta(1) needs to become datetime.datetime(2002, 10, 26, 2, 0) (Note the hour change)

I was thinking in calendrial arithmetic, which the datetime module doesn't need to care about.

On Wed, Apr 15, 2015 at 12:59 AM, Stuart Bishop <stuart at stuartbishop.net> wrote:

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 isdst 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), isdst=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 isdst=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 isdst=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/

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