[tz] Converting cities to tz identifiers (tangent)

John Hawkinson jhawk at MIT.EDU
Tue Feb 20 23:23:14 UTC 2018

(multiple replies inline)

Guy Harris <guy at alum.mit.edu> wrote on Tue, 20 Feb 2018
at 15:04:11 -0800 in <91FA6B60-4729-4B8D-80F9-2E79851A8D76 at alum.mit.edu>:

> What is meant by "the US Eastern time zone"?  The zone that, barring
> adjustments such as Daylight Savings Time, is 5 hours west of UTC -
> and that may include locations that do make DST adjustments and
> locations that don't?

I think it generally means the region as defined in 15 USC 260a et seq.

> If so, then all that asking for "the US Eastern time zone" will get
> you is "5 hours west of UTC"; it won't get you any rules for
> adjustments, because not all locations in the Eastern time zone will
> necessarily follow the same rules.

Well, that's certainly not true in the tz project. It'll get you America/New_York, which gets you the NYC rules. (Now, maybe that is what's wanted, maybe it's not. In my case particular case, it's fine, because I'm not concerned with history.)

> The tzdb is not currently the place to ask about "the US Eastern
> time zone",

That is not the question I'm asking, however:

> because it doesn't have any mechanism to ask what the
> UTC offset of the "US Eastern time zone" is.

Sure it does. The answer is just more approximate than some people (you?) might like, but is perfectly fine for others' purposes (mine own).

Paul Eggert <eggert at cs.ucla.edu> wrote on Tue, 20 Feb 2018
at 14:25:49 -0800 in <5b1e840e-b46d-9406-820a-754304e48e60 at cs.ucla.edu>:

> It depends on how portable you want to be. Some implementations do

Portability is an intersting question. Yet another thing to balance with what I called "stability." Like most people, I'm at present only concerned with portability to the systems I care about, which doesn't have to include FreeBSD right now, although I am definitely annoyed at Garrett's unilateral decision to make my preferred form less portable than I would like it to be :)

> If New York ever switches to Atlantic time (unlikely but conceivable
> given the current state of US politics), we'll link some other
> location to US/Eastern in the 'backward' file. Although it's not now
> clear what that other location would be (it'd depend on what part of
> the US switches), one candidate would be
> America/Indiana/Indianapolis.

I'm pretty sure that's not a reasonable candidate :)

I can't tell if you're joking, but even if the rules in Theory happen to allow or recommend that choice, I'm pretty sure we would revise them to discourage choosing a candidate that has a history that is so radically divergant from the vast majority of the geographical area in the zone.

--jhawk at mit.edu
  John Hawkinson

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