Time Zone naming

Martin Barnes barnes at yahoo-inc.com
Fri Dec 5 17:18:08 UTC 2008

Thanks Scott. Yes, I better understand the provenance and status of the
Olsen data now.

I will look into the details to see the efficacy of constructing the
roll-up zones. Since they will be neither an Olsen zone nor a CLDR
metazone, we may have issues applying a name. The resultant data will be
predominantly Olsen with a few roll-ups but we would want to use the
Olsen name throughout. Reflecting on this, I think that it would be
prudent to align to the Olsen zones otherwise we might invite problems
by associating a place to an Olsen time zone name, when it actually
belongs to a zone that has been rolled-up into a "superzone" for which a
different name has been applied.


I completely understand that the Olsen time zones change from time to
time. New ones are added and existing ones may be modified. 

My question is...is there anyway of being notified when any changes
occur to the tz database or is it a matter of systematically checking?





From: Scott Atwood [mailto:scott.roy.atwood at gmail.com] 
Sent: 04 December 2008 20:05
To: Martin Barnes
Cc: tz at elsie.nci.nih.gov; tz at lecserver.nci.nih.gov
Subject: Re: Time Zone naming


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 body.


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>

	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.




	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.





		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
		Do these combined "super" zones exist? If so, is there
		available that indicates how the individual zones roll
		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

	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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