[tz] tzdb timezone names/identifiers and links

Philip Paeps philip at trouble.is
Thu Feb 21 03:44:40 UTC 2019

On 2019-02-21 12:27:50 (+0900), Brian Inglis wrote:
> On 2019-02-20 16:13, Michael Douglass wrote:
>> On 2/20/19 17:50, Guy Harris wrote:
>>> On Feb 20, 2019, at 2:34 PM, Clive D.W. Feather wrote:
>>>> Guy Harris said:
>>>>> Maybe this is *another* case where "some applications "misuse" 
>>>>> tzdb identifiers" and where it might "be better to solve this on a 
>>>>> more general level such as geo-location<=>timezone mappings", e.g. 
>>>>> a meeting date shouldn't be tagged with {date/time, tzdb 
>>>>> identifier} but with {date/time, geographic coordinates}.  I'm 
>>>>> not sure whether the geographic coordinates of the meeting 
>>>>> location are more or less likely to change than the time zone 
>>>>> rules of that location,
>>>> That doesn't always solve the problem; it can make it worse.
>>>> For example, I have a meeting which *I* take part in at my 
>>>> location, but is booked for 07:00 Seattle time, not 15:00 UK time. 
>>>> I don't want it tagged with my location.
>>> If that is a phone or online meeting, rather than an in-person 
>>> meeting, it doesn't have *a single* location, right?  At *most*, it 
>>> has a "central" location, where the people coordinating the meeting 
>>> reside, or something such as that, but that's not the location where 
>>> the meeting is occurring.
>>> If the expectation is that the calendar software will automatically, 
>>> through the Magic of the TZDB, show everybody the meeting time at 
>>> their location, then it's going to have to keep track of the 
>>> locations of all the participants, so it'll have to get tagged with 
>>> the location of the (presumed) people in Seattle attending it, and 
>>> the location of the people in the UK attending it, etc., so that the 
>>> meeting might be tagged with the "central" location, *and* each 
>>> participant would be tagged with their location at the time of the 
>>> meeting.
>> Absolutely - one of the intentions of the extensions to the calendar 
>> specs.
>> In this age of online meetings the attendees (intended) location - at 
>> the time of the meeting - becomes much more important.
>> The location of the meeting itself then becomes a matter of what is 
>> most appropriate - perhaps the organizers location or a room if some 
>> people are local.
> My expectation is that the meeting organizer schedules an event at 
> their reference location(s), in those locations' time zones, and 
> attendees may display the event in the time zone for their default, 
> current, or other location.

That seems to be the way things work in my experience in the "real 

Organiser: "We are having a meeting next Tuesday at 18:00 New York 
time".  Regardless of where everyone else in the world happens to be.  I 
expect calendar software to make a note of the "New York time" and alert 
me around 17:30 New York time, even if I happen to be in Hong Kong at 
the time (and probably wishing to be asleep).

And in the real world, that has to be robust in the face of daylight 
saving and other silliness.  If New York decides to move to a different 
UTC offset the night before, but Hong Kong doesn't, I still want to be 
alerted at 17:30 New York time.  Even if that's at a different offset 
than it was when the meeting was originally scheduled.

> When I say locations, that allows for an event being a flight or trip 
> from one location to another in a different time zone.

It also allows one to schedule an event in a third location.  E.g. you 
schedule a meeting at 18:00 next Tuesday in New York while you are in 
Hong Kong knowing that you'll be in London the day of the meeting.  
Regardless of where you happen to be, you still want the alert when 
people in New York agree it's 17:30 on Tuesday.

> My expectation is that I can associate each location with a time zone 
> for an extended period of time, and if the time zone in effect at that 
> location changes, I can change that association, effectively providing 
> my own gazetteer of time zones for locations of interest to me.
> Larger organizations may provide and update gazetteers with their 
> systems so that I need not associate time zones with locations.

Precisely.  Users don't want to care about that.  Users also don't want 
to have to worry about the time differences changing for whatever reason 
-- whether daylight saving or political meddling.  Users just want the 
alert half an hour before the meeting, whenever that happens to be.


Philip Paeps
Senior Reality Engineer
Ministry of Information

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