[tz] Preparing to fork tzdb

Guy Harris gharris at sonic.net
Wed Sep 22 23:53:47 UTC 2021

On Sep 20, 2021, at 1:06 AM, Stephen Colebourne via tz <tz at iana.org> wrote:

> As most of you probably know, there is a dispute about the tzdb
> maintainer's recent changes to merge large numbers of time-zones
> [1][2]. These have the effect of wiping out historic time-zone
> information on many locations where the data has been in tzdb for many
> years.

The comment at the beginning of 2021a's backzone says:

# From Paul Eggert (2014-10-31):

# This file contains data outside the normal scope of the tz database,
# in that its zones do not differ from normal tz zones after 1970.
# Links in this file point to zones in this file, superseding links in
# the file 'backward'.

# Although zones in this file may be of some use for analyzing
# pre-1970 timestamps, they are less reliable, cover only a tiny
# sliver of the pre-1970 era, and cannot feasibly be improved to cover
# most of the era.  Because the zones are out of normal scope for the
# database, less effort is put into maintaining this file.  Many of
# the zones were formerly in other source files, but were removed or
# replaced by links as their data entries were questionable and/or they
# differed from other zones only in pre-1970 timestamps.

so it's not as if this is the first time that historic information has been moved to backzone, which, although not "wiping it out" in the sense of making it no longer exist in any form in the tzdb repository or releases, removes it from any collection of compiled tzdb files not build using backzone, and removes it from any data set that applications get by reading tzdb source files and not reading backzone.

One example of data that's in backzone in 2021a is the data for America/Montreal, the comment for which in 2021a backzone is:

# Canada
# From Paul Eggert (2015-03-24):
# Since 1970 most of Quebec has been like Toronto; see
# America/Toronto.  However, earlier versions of the tz database
# mistakenly relied on data from Shanks & Pottenger saying that Quebec
# differed from Ontario after 1970, and the following rules and zone
# were created for most of Quebec from the incorrect Shanks &
# Pottenger data.  The post-1970 entries have been corrected, but the
# pre-1970 entries are unchecked and probably have errors.

The example that's being cited as a problem with the merge is Europe/Oslo, the comment for which in 2021a europe is:

# Norway
# http://met.no/met/met_lex/q_u/sommertid.html (2004-01) agrees with Shanks &
# Pottenger.

So one difference here appears to be that the pre-1970 data for Europe/Oslo may be accurate while the pre-1970 data for America/Montreal may be inaccurate.

Another difference is that Toronto and Montreal are in the same country, while Oslo and Berlin aren't.


	Were there any complaints when America/Montreal was turned into a link in 2015c (if the NEWS file is to be believed)?

	Is the concern that there's software, processes, etc. that depend on Europe/Oslo having the correct pre-1970 data, because that data appears to match what The Norwegian Meteorological Institute says, whereas there may be less software, processes, etc. that depend on America/Montreal having pre-1970 data because that data might be bogus?

More information about the tz mailing list