Time Zone naming
barnes at yahoo-inc.com
Thu Dec 4 18:48:17 UTC 2008
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
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
"Olsen" name where multiple zones exhibit the same "behaviour"
of belonging to the same country, having the same UTC offset and
the same DST rules.
For example, it appears that all clocks within all locations
Argentina will have the same time all year round. The 12 zones
the same behaviour. The same is true of China and a number of
I have been aware of the concept of a "consolidated" or
zone which is a combined zone that takes the name of the most
location (eg. "America/Buenos_Aires" in the case of Argentina)
Do these combined "super" zones exist? If so, is there
available that indicates how the individual zones roll up?
My enquiry relates to a need to provide information that can
the correct timezone for every place (city, postcode, county,
etc) on earth via a back-end mapping service that calculates the
relationship between the place coordinate and the timezone
I am looking to build up an accurate timezone boundary map
using existing map objects as building blocks.
Yahoo! Geo Technologies
Geo Informatics team
Cycle tracks will abound in Utopia. ~H.G. Wells
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