[Python-Dev] Status on PEP-431 Timezones

Terry Reedy tjreedy at udel.edu
Mon Jul 27 20:55:53 CEST 2015


On 7/27/2015 3:09 AM, Tim Peters wrote:
> [Paul Moore <p.f.moore at gmail.com>]
>>> ....
>>> As an example, consider an alarm clock. I want it to go off at 7am
>>> each morning. I'd feel completely justified in writing
>>> tomorrows_alarm = todays_alarm + timedelta(days=1).
>
> [Lennart Regebro <regebro at gmail.com>]
>> That's a calendar operation made with a timedelta.
>
> It's an instance of single-timezone datetime arithmetic, of the
> datetime + timedelta form.  Your examples have been of the same form.
> Note that after Paul's
>
>       tomorrows_alarm = todays_alarm + timedelta(days=1)
>
> it's guaranteed that
>
>      assert tomorrows_alarm - todays_alarm == timedelta(days=1)
>
> will succeed too.
>
>> The "days" attribute here is indeed confusing as it doesn't mean 1 day,
>> it means 24 hours.
>
> Which, in naive arithmetic, are exactly the same thing.

I think using the word 'naive' is both inaccurate and a mistake.  The 
issue is civil or legal time versus STEM time, where the latter includes 
applications like baking cakes.  It could also be called calendar time 
versus elapsed time.  (Financial/legal arithmetic versus STEM arithmetic 
is a somewhat similar contrast.)

The idea that an hour can be sliced out of a somewhat random March day 
and inserting it into a somewhat random October day is rather 
sophisticated.  It came from the minds of government bureaucrats.  It 
might be smart, dumb, or just a cunning way for civil authorities to 
show who is in charge by making us all jump.  But not 'naive'.

'Naive' means simple, primitive, or deficient in informed judgement. It 
is easy to take it as connoting 'wrong'.  Tim, you have been arguing 
that civil/legal time arithmetic is not naive. Calling civil time naive 
undercuts this claim.

-- 
Terry Jan Reedy



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