Time Zone naming

Scott Atwood scott.roy.atwood at gmail.com
Thu Dec 4 20:05:03 UTC 2008

The Olson zoneinfo database and CLDR are the closest thing that presently
exist to a standard for timezones, but they are de facto standards, not de
jure standards administered by any kind of official international standards

If it would be useful for your internal purposes, there is certainly enough
information in the Olson data to construct "roll-up" timezones for yourself
where the ISO-3166-2 code, GMT offset, and DST rules are all presently
identical.  But as I mentioned below, you should expect such entities to be
rather fluid, and you should prefer to base your implementation on the full
fidelity Olson timezones.  Such "roll-up" timezones would be finer grained
entities than CLDR metazones, since the only requirement for metazones is
that they must have the same display labels.  Individual Olson timezones
within a CLDR metazone may have different DST rules and may belong to
different regions.

Also, on the topic of future proofing, you should be prepared for the fact
that new Olson timezones are added from time to time whenever a subregion of
an existing zone changes its time definition independently of the rest of
that subregion.  This happened during the recent timezone change in
Argentina where America/Argentina/Salta split off from
America/Argentina/Cordoba.  Any Olson update could thus potentially require
updates to your boundary definitions.


On Thu, Dec 4, 2008 at 10:48 AM, Martin Barnes <barnes at yahoo-inc.com> wrote:

>  Thanks Scott. This is very useful insight.
> I am really only ever concerned with expressing the current timezone for
> any given place and less worried by any need to relate to any historical
> dates and times.
> Consequently it was my hope to use a sanctioned standard that identified
> timezones as regions with the same political definition (ISO region code),
> the same UTC offset and the same DST rules but that ignored any historical
> context; whereby I could find and group all places that exhibited and
> experienced the same current time behaviour (such as all the cities in China
> belonging together).
> But it appears that no such standard exists. Furthermore, and as you point
> out in the Argentina example, using the "pure" Olsen is recommended in view
> of the frequent changes that are both "official" as decreed by governments
> and "unofficial" as adopted by local usage. Having these zones pre-defined
> as boundary map objects allows us a better degree of future-proofing against
> changes to come. That is good enough for me.
> I guess if I need to I might be able to create relationships between zones
> that behaved the same at any given point in time I might want to look at the
> CLDR metazones, albeit with some caution.
> Thanks for your time and help.
> -Martin
>  ------------------------------
> *From:* Scott Atwood [mailto:scott.roy.atwood at gmail.com]
> *Sent:* 04 December 2008 17:24
> *To:* tz at elsie.nci.nih.gov
> *Cc:* tz at lecserver.nci.nih.gov; Martin Barnes
> *Subject:* Re: Time Zone naming
> To the best of my knowledge, the Olson database itself does not define any
> kind of "roll-up" timezones.  The closest thing I am aware of is the CLDR
> concept of "metazones" which group together Olson timezones that share a
> common display string, like "Eastern Standard Time".  However, I believe
> these metazones can include timezones that have different DST rules.
> Rather than try to use "roll-up" timezones, from personal experience, I
> would urge you to use the full Olson timezone list if possible.  World
> timezone rules are highly dynamic and change with surprising frequency.  And
> it is not uncommon for two Olson timezones to have the same GMT offset and
> DST rules in one release of Olson, but then have different rules in a future
> release.   The example that you cite, Argentina, is an excellent example.
>  Until just a few weeks ago, all of Argentina was effective under the same
> set of time zone rules, but when the central government decided to observe
> DST this year, several of the states decided to remain in standard time.  An
> application that had assigned "America/Argentina/Buenos_Aires" to everyone
> in Argentina regardless of their actual Olson timezone would have broken.
> Also note that it can be useful to maintain the separate timezones if your
> application needs to format and display historical dates and times, as in
> logging or transaction history.  Timezones that have the same GMT offset and
> DST rules today may have had different rules in the past, and having the
> most accurate timezone means you could display the historical records
> correctly as well.
> -Scott
> From: Martin Barnes [mailto:barnes at yahoo-inc.com]
> Sent: Friday, November 28, 2008 11:48
> To: tz at lecserver.nci.nih.gov
> Subject: Time Zone naming
> I have a question related to the accepted standard for expressing the
> "Olsen" name where multiple zones exhibit the same "behaviour" in terms
> of belonging to the same country, having the same UTC offset and exactly
> the same DST rules.
> For example, it appears that all clocks within all locations within
> Argentina will have the same time all year round. The 12 zones reveal
> the same behaviour. The same is true of China and a number of other
> countries.
> I have been aware of the concept of a "consolidated" or "preferred" time
> zone which is a combined zone that takes the name of the most important
> location (eg. "America/Buenos_Aires" in the case of Argentina)
> Do these combined "super" zones exist? If so, is there information
> available that indicates how the individual zones roll up?
> My enquiry relates to a need to provide information that can identify
> the correct timezone for every place (city, postcode, county, state, etc
> etc) on earth via a back-end mapping service that calculates the spatial
> relationship between the place coordinate and the timezone boundary.
> I am looking to build up an accurate timezone boundary map essentially
> using existing map objects as building blocks.
> Many thanks
> -Martin Barnes
> ______________________
> GeoData Manager
> Yahoo! Geo Technologies
> Geo Informatics team
> London
> --
> Scott Atwood
> Cycle tracks will abound in Utopia.  ~H.G. Wells

Scott Atwood

Cycle tracks will abound in Utopia.  ~H.G. Wells
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