Which tzdata zones have complete pre-1970 history?
alois at astro.ch
Wed Aug 17 11:29:16 UTC 2011
With some colleagues who are like myself interested in a complete
pre-1970 timezone history in a tzdata compatible file format, I have
started a project whose goals is the following:
a) complete existing tzdata zone files with incomplete pre-1970 history,
so that they become historically complete.
b) create additional tzdata zone files which represent zones where
tzdata currently fails to differentiate between areas with different
The motivation for this effort is that astrology needs complete timezone
histories, for each point on the globe where registered birth data are
likely to exist.
I am aware that the design goal of tzdata is only to represent complete
and correct timezone information beginning with 1 January 1970.
I do not question this design goal. Still, there is a community of
tzdata users who need more than that, and who are missing an open source
database with complete timezone histories.
For example, Switzerland needs an extra zone Europe/Geneva to represent
Geneva timezone between 1851 and 1894.
Germany needs about 13 different zones to represent the timezone history
of different parts of the country. The matter there is even mor
complicated because Germany changed size repeatedly in the 19th and 20th
century, parts were added or removed from it and assigned to other
By the way, it is excellent that tzdata zone names are based on cities
and not on countries, as this avoids a lot of political issues.
I think it would be a good start investigating for which of the approx.
400 tzdata zones the pre-1970 history can be considered complete and
We have no absolute reliable measure what 'complete and correct' means.
But a comparison with a recent revision of Shanks/Pottenger American and
International atlas would already serve to a large extent.
As a working definition I propose:
Those tzdata zones whose history details are equivalent or better than
the corresponding history lists by Shanks & Pottenger should be
considered complete and correct.
Switzerland is not historically complete in tzdata.
tzdata zone Europe/Zurich
is correct, but a separate zone Europe/Geneva is missing.
The canton of Geneva had Geneva mean time 6E0915 between 1851 and the
introduction of standard time on 1 June 1894
(SOURCE: GABRIEL, TRAITE DE L'HEURE DANS LE MONDE, edition 1991)
Shanks/Pottenger have a separate table for this canton, but it contains
a small error as well: it assigns LMT until 1894, while in fact it
should be Geneva city observatory mean time (24m37s east of Greenwich)
between 1851 and 1894. The error is small because this canton is not
big, and the most distance any other town has from the Geneva meridian
is 4 minutes of arc.
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