[Datetime-SIG] PEP-0500 (Alternative datetime arithmetic) Was: PEP 495 ... is ready ...

[Chris Barker - NOAA Federal]

where is
>> a bug in the following calculation?
>>
>> >>> print(lt)
>> 2015-03-07 12:00:00-05:00
>> >>> lt += timedelta(1)
>> >>> print(lt)
>> 2015-03-08 12:00:00-05:00
>
> Well, let's say I live in New York, so all winter long I've been seeing things like "2015-01-17 9:37:51-05:00", etc. and then the time switches in March and I fail to notice that the "-05:00" is still "-05:00" and not "-04:00" -- especially since my watch, clock, smart phone, etc., don't display the offset -- well, something bad will happen:

If you expect to see local time, you probably aren't looking at ISO timestamps.

> exactly what depends on what the user was expecting when adding a day.

Not really -- that distinction is about Duration vs Period arithmetic.
This is about the fact that a tzinfo object that represents an offset
simply has no idea about DST transitions -- it CAN'T do the period
arithmetic some people want.

In your use case -- don't use an offset. Period, end of story.

If you want Period arithmetic, use full featured tzinfo objects. ( and
dateutils ). And don't expect to be able to round-trip via an ISO 8601
string.

-Chris