Time Zone naming
scott.roy.atwood at gmail.com
Thu Dec 4 17:23:43 UTC 2008
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.
> 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
> 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
Cycle tracks will abound in Utopia. ~H.G. Wells
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