Subject: [Tzdist] Data identifiers

From: Lester Caine <[email-masked]>
Date: Sat, 04 Oct 2014 11:38:10 +0100

While this is outside the scope of tzdist, it is tied up with making
tzdist function in the future.

Currently there is only one set of data for Europe/London and a number
of aliases which cross reference that base set. The backzone file
provides documented corrections to the aliased identifiers prior to
1970, and so while for a recent truncation the data is identical, the
accuracy falls off going back in time. What is missing from even the
backzone file is the admittedly narrow window of events between 1840
when railways started to standardise time on stations, and 1880 or so
when the rest of the country joined the railways. I believe other
countries in Europe had a similar path to standardisation?

TZ quite rightly does not want to have to worry about Europe/UK/Oxford ,
Europe/UK/Exeter or Europe/UK/GreatWesternRailway time, and that is
quite understandable. So we need an alternate publisher willing to
provide the data. The problem I still see is dovetailing data to provide
a consistent view so that TZ can be a source from 1970 on, or some
earlier fixed point if the discussions there can agree on a better way
of handling the more global historic TZ data, and the historic material
that pre-dates that.

Europe/UK/Oxford becomes Europe/London in 1880, but Europe/Isle_of_Man
did not 'join' until 1883. On one hand this is totally irrelevant to a
large majority of users, but it is important when working with the
growing volume of on line material from that time.

This is a bit like the debate on 'deleted' timezones. Europe/Isle_of_Man
is currently in that limbo since it appears to be Europe/London for all
time which is not correct, and it's real truncation date is 1883 prior
to which 'Tynwald' time is a better approximation but for which further
research is now needed. In essence Europe/Isle_of_Man is a deleted set
of data currently!

There does need to be some statement in tzdist that requires that the
initial implementation WILL only be a truncated data set. That matches
the statements about the accuracy of the TZ data and the lack of a
reliable un-truncated data source? That TZ data can reliably used prior
to the 1970 cut off is a discussion for the TZ list, but I would request
that until an alternative reliable date is available that should be the
point for this first implementation.

-- 
Lester Caine - G8HFL
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