[Datetime-SIG] Are there any "correct" implementations of tzinfo?

Tim Peters tim.peters at gmail.com
Mon Sep 14 21:15:12 CEST 2015

>> Wouldn't it be sufficient for people in Creighton to set their timezone to
>> US/Central? IIUC the Canadian DST rules are the same as the US ones. Now,
>> the question may remain how do people know what to set their timezone to.
>> But neither pytz nor datetime can help with that -- it is up to the
>> sysadmin.

[Laura Creighton]
> The Canadian DST rules are not the same as the US ones, wherein lies
> the rub.

?   All US locations observing DST switch at 2AM on the local clock on
the 2nd Sunday of March and the 1st Sunday of November (since 2007).
Best I can tell, exactly the same is true of all zones observing DST
in Canada and in Mexico.

  The province of Saskatchewan has a few areas which prefer to
> be on Mountain time, which we will ignore in this mail.  The bulk of
> them prefer to be on Central time, which is the same as Winnipeg and
> Chicago.  But what happens on DST start day (early March) in
> Saskachewan?  Provincial law mandates that you do not change your
> clock, and do not adopt daylight savings time.  The people in
> Saskachewan thus stick to CST all year round.  And this is how
> sites like http://www.timeanddate.com/worldclock/canada/saskatoon
> record things, with them at CST.

I'm sure Guido is only talking about places in Canada that _do_
observe DST.  There are also places in the US that don't observe DST.
Of course "the DST rules" only apply to locations that do observe DST,
and that's what Guido is asking about.

> However, the people of Creighton have decided to keep DST, in
> violation of the law, so they set their clocks forward.
> http://www.timeanddate.com/worldclock/canada/creighton does this
> properly as well, showing them with CDT.
> But this is not quite the complete story.  In many (most?) places in
> Saskatchewan, the rule is understood differently.  Instead of 'we keep
> to CST all year long' is is understood that  'we keep central time in
> the winter and mountain time in the summer'.
> This makes parsing log files from all over Saskachewan, where sometime
> in March things often stop saying CST and say MDT instead rather more
> interesting than the adverage person might suspect.

Read question as "in general" ;-)  There are plenty of exceptions to
the general rules all over the world.

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