[Python-Dev] TZ-aware local time

[Guido van Rossum]

On Tue, Jun 5, 2012 at 6:39 PM, Alexander Belopolsky
<alexander.belopolsky at gmail.com> wrote:
> On Tue, Jun 5, 2012 at 9:16 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> ...  Local time should only be used for displaying
>> dates and times to humans (since we care about little things like
>> local sunrise and sunset, local business hours, etc) and for
>> inter-system coordination where such details are relevant.
>>
>
> Displaying local time would be addressed by what I called the first
> feature: given the timestamp find the local time and the TZ
> name/offset:
>
> $ date
> Tue Jun  5 21:28:21 EDT 2012
>
> Most humans will ignore the TZ information, but the format above can
> represent any moment in time unambiguously.
>
> A related but different problem is taking time input from humans.  Air
> traffic control systems may handle all times in UTC, but when I search
> for my flight, I enter local time.  There may be two flights one at
> 1:30 AM EDT and the other at 1:30 AM EST.  In this case I need some
> way to figure out which one is mine and I will look at the TZ part.
> (Hopefully the user interface will present a helpful explanation of
> what is going on.)
>
>
>> The datetime module should be designed to make this *as easy as
>> possible*. Adding a "local time" tzinfo object (with the ambigous hour
>> favouring the non-DST time, and the missing hour triggering an error
>> at construction time) would be a good step in the right direction: it
>> allows local times to be clearly flagged, even though they're
>> explicitly *not* appropriate for many kinds of processing and need to
>> be converted to a more suitable format (such as a naive datetime
>> object, or one with the timezone set to UTC) first.
>
> This is exactly my proposal, but it does not help when you need to get
> on the right 1:30 AM flight.

Trust me. Even if there was only one 1:30AM flight everybody including
the crew would have trouble getting there on time. There's a reason
why the DST change happens around 1AM.

(Unrelated fun DST fact: there's a difference between the way Europe
and the US coordinate DST changes across multiple timezones. In the US
each timezone switches at 1AM local time, making the difference
between adjacent timezones vary for an hour. In Europe the linked
timezones all switch simultaneously, keeping the zone differences in
sync but making the local time of the switch vary.)

-- 
--Guido van Rossum (python.org/~guido)