[tz] draft of change summary for next tz release

Zoidsoft zoidsoft at gmail.com
Wed Sep 18 14:02:18 UTC 2013

The use I have for the data are primarily for accurate display of the sky
at any given moment which the post 1970 data does very well.  Astronomers
avoid using zones generally and use UTC or GMT/LMT for past events but
there are some programs such as Starry Night and Skymap (any my
software) that use the PC's clock and a zone offset (which can be set to
past dates researching such phenomena as the timing and accuracy of
occultations with select planetary perturbation terms, etc).  Astrologers
(not astronomers) have been the primary source for this older information
and anyone who wants to display the sky with only the knowledge of what
time is on their watch would be interested in this historical data.  While
the time zone information of the past (particularly the first half of the
20th century) is largely suspect (and really anyone who has done even
modest research can tell this), it is important to anyone with this
interest.  Since the historical data doesn't change much, I maintain a
separate database for this info (with warnings) since the tzdb can't
promise that it will always be there and many have no interest in this type
of use.

One might want to keep in mind that there are many who have written access
to the tzdb in other languages.  Here is one example:


To some extent the historical usage accuracy issue is offset by the fact
that the particular use I describe above has worn grooves into the more
densely populated areas which has a tendency to be more accurate than
outlying areas.

On Wed, Sep 18, 2013 at 8:22 AM, Stephen Colebourne <scolebourne at joda.org>wrote:

> On 18 September 2013 10:59, Meno Hochschild <mhochschild at gmx.de> wrote:
> > So I support the removal, since I think the discussed data obviously
> appear
> > to be of very questionable nature. We cannot even consider the discussed
> > data as "our best estimate".  Of course, there is no 100% guarantee - no
> > black and white. If someone can know it better then it is easy to add the
> > lost data again. But we should really not let the users in the state of
> > wrong assumptions. Stability of data should not be the primary concern,
> > rather correctness. And most users (near 100%) don't care about old that
> is
> > to say archeological timezone data.
> Pre-1970 data matters to some more than others. I can see a range of
> positions:
> a) delete all pre-1970 data
> b) only have Zones for areas distinct after 1970, other IDs are Links,
> full data where available for each Zone
> c) only have IDs for areas distinct after 1970, full data where
> available for each ID
> d) create new IDs where data only differs before 1970
> I'm arguing for (c), which I previously believed was the tzdb's goal.
> The data deletion is based on (b).
> The quality of data deleted is also of different value to different
> people. I'll try to explain it in a different way...
> We know that the quality of the historic data for the Carribean is
> dubious. Lets give it an accuracy rating of 20%. One argument is that
> removing data with 20% accuracy is a good thing, and that is an
> understandable position. However, its important to look at the
> consequences of the deletion. Previously, location A (eg. Guadeloupe)
> had a 20% accuracy rating for its historic data. After the change it
> still has a 20% accuracy rating just with different data. But, that
> 20% accuracy rating refers to location B (eg. Port of Spain). From the
> perspective of location A, the accuracy is now lower, say 5%, because
> 20% accurate data for location B clearly is even less accurate for
> location A.
> I understand that the distinction here is fine. But its rather like
> saying "we published a guess 10 years ago for when the first factory
> opened in Brussels, but it now OK to replace that data with the guess
> we made for when the first factory opened in London" (assuming we
> recorded factory opening dates). I value each guess being distinct for
> each location in the absence of better information.
> Stephen
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