Time Zone naming

Mon Dec 8 10:41:40 UTC 2008

Thanks Scott

Please could I and my colleague, Robert Halliday (emailed here) be added
to the tz mailing list?

It would be good to have prior notice of upcoming changes and be able to
have the opportunity to discuss how time zone boundaries are affected.

Many thanks for all your help.

-Martin 

________________________________

From: Scott Atwood [mailto:scott.roy.atwood at gmail.com] 
Sent: 05 December 2008 18:11
To: Martin Barnes
Cc: tz at elsie.nci.nih.gov; tz at lecserver.nci.nih.gov
Subject: Re: Time Zone naming

The best way to find about changes to the tz database is to subscribe to
and monitor the tz mailing list:  tz at elsie.nci.nih.gov.   Upcoming
changes are typically discussed in advance of their release.

-Scott

On Fri, Dec 5, 2008 at 9:18 AM, Martin Barnes <barnes at yahoo-inc.com>
wrote:

	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?

	-Martin

________________________________

	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.

	-Scott

	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

-- 
Scott Atwood

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

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