[Zope3-dev] Re: [Python-Dev] Holes in time

[Aahz]

On Thu, Jan 02, 2003, Guido van Rossum wrote:
>Aahz:
>>
>> I'm confused: are you saying that Guido's calendar doesn't get
>> internally converted to UTC?
> 
> That's correct.  I use a Palmpilot, which has no knowledge of
> timezones or DST at all.  In fact, I write down appointments in
> whatever local time I will be in on a specific date, i.e. if I were to
> fly to California to meet with you at 1pm on some date, I'd put that
> down for 1pm on that date; on the plane I'll set the Palmpilot's clock
> back three hours.
> 
> I've heard of a calendar application on Windows (Outlook?) that
> records all your appointments in UTC and displays them in local time.
> That means that when you are temporarily in a different timezone and
> you change your computer's clock to match local time, all your
> appointments are displayed in that local time -- which means that if
> you want to look up an appointment next week when you're back home, it
> is shown wrong!  (E.g. a 1pm appointment in Washington DC would show
> up as 10am when the user is in San Francisco -- very confusing when
> you're in SF talking on the phone to a potential customer on the east
> coast!)

All that is true and makes perfect sense -- but then Tim's comment about
converting your calendar to someone else's wall clock makes no sense.

>> If not, then the chart from your message starting this thread
>> doesn't apply, because here you're talking about converting from
>> wall clock to wall clock, which I think is a conversion that makes
>> no sense -- that perhaps would be a suitable occasion for an
>> exception.
> 
> You can get from wall clock + tzinfo to UTC quite easily using the
> utcoffset() method of the tzinfo -- that's what it is for.  So wall
> clock to wall clock conversion reduces to UTC to wall clock
> conversion anyway.

No, because there's no information about *which* wall clock is being
used.  As you yourself said above, you casually enter times for
appointments in the local time that you expect to use them, without
recording the extra information needed to make conversions.

>> If Guido's appointment does get converted to UTC, then there's always a
>> sensible (if possibly ambiguous) conversion to wall time, and if he
>> looks at his calendar, he should be able to see if his appointment got
>> converted to UTC correctly.
> 
> No -- why would I care about UTC or even know the UTC offset of my
> wall clock time?  If I make an appointment for early next April, why
> should I have to know whether DST applies or not at that particular
> date?

You don't -- but when your application displays wall time, you'd see a
discrepency from the time you entered.  This only applies when there's a
consistent system of timezone information.

You can't do date/time arithmetic and conversions with pure wall clock.
Period.  Python should raise exceptions if you try.
-- 
Aahz (aahz@pythoncraft.com)           <*>         http://www.pythoncraft.com/

"There are three kinds of lies: Lies, Damn Lies, and Statistics."  --Disraeli