[tz] Adding verified historic details

Russ Allbery rra at stanford.edu
Wed Sep 4 22:13:07 UTC 2013

Lester Caine <lester at lsces.co.uk> writes:

> A proposal has been put forward to prevent the creation of new timezones
> just for different locations prior to standard time being
> adopted.

Just to be clear, this isn't really a proposal; rather, this is the
existing maintenance practice for the tz database that's been followed
fairly consistently for many years now.  In other words, it's the existing
standard, not the proposal.  Therefore, to be pedantically correct, the
proposal (i.e., the change from existing practice) is to permit
introduction of new zones that differ only in pre-1970 dates, something
that the tz project has not previously done.

> If there is documentary evidence of a different time pattern such has
> been added for the Isle of Man and is about to appear for the channel
> islands then it should be allowed not blocked.

There are certainly several people here advocating for this (new!)
practice, and the packrat in me that likes having as much data as possible
approves of the idea of incorporating whatever data people are willing to
do the work to research, at least if it's practical to do so.

(I question whether it's practical to represent some of the historic time
practices, such as using one time zone inside railway stations and a
different one for municipal time, but we can cross that bridge when
someone has time to put forward a complete proposal for how it would be
done.  If anyone ever does.)

However, this is a divergence from existing practice, and it sounds like
Paul would prefer to have a winnowing mechanism in place before changing
that maintenance rule.  That seems like an entirely reasonable position to
me.  We haven't had or tracked that data for years; waiting a few more
months (if that) until a winnowing process can be properly reviewed and
adopted isn't going to hurt anything.  And, in the meantime, the data that
people are gathering can be documented in the form of patches here or Git
branches or a number of other methods, so it won't be lost when there is a
project in a position to adopt it.

> Where different locations adopt standard time at different documented
> times then the source file format needs to be change to accommodate
> that. I proposed that the location and date is recorded and then links
> to the following timezone for the rest of the data. The Isle of man is a
> perfect example of this since all it differs in is the date of the start
> of using GMT.

This sounds like a good idea to me, although I don't know what
difficulties it poses for the rest of the software.  It would indeed be
nice to not have to duplicate all the rules into a separate zone when the
zones are identical after a certain date, and it also seems like it would
make some possible winnowing strategies more straightforward.

Russ Allbery (rra at stanford.edu)             <http://www.eyrie.org/~eagle/>

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