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

Lennart Regebro regebro at gmail.com
Tue Apr 14 21:04:48 CEST 2015

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.

That means that:

tz = get_timezone('US/Eastern') dt = datetime(2002, 10, 27, 1, 0, tz=tz, is_dst=False) dt2 = dt - 420 + 420 assert dt == dt2

Will fail, which will be unexpected for most people.

I think there is no way around this, but I thought I should flag for it. This is a good reason to do all your date time arithmetic in UTC.

//Lennart

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