[tz] Issues with pre-1970 information in TZDB

[Guy Harris]

On Sep 22, 2021, at 3:23 PM, Howard Hinnant <howard.hinnant at gmail.com> wrote:

> On Sep 22, 2021, at 5:17 PM, Tom Lane via tz <tz at iana.org> wrote:
> 
>> One thing that's particularly sticking in my craw about the
>> changes under debate is that they undeniably made tzdb worse as a
>> description of reality in the places at issue.
> 
> This is my objection as well.  Before the change the tzdb does a “best effort” on pre-1970 dates.

"Best" in what sense?

The 2021a data makes an effort to keep the data for Europe/Oslo separate from the data for Europe/Berlin.  It makes no effort to keep the data for America/Montreal separate from the data for America/Toronto.

The comment in backzone about the America/Montreal data 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 comment in the 2021a europe about the Europe/Oslo data is

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

so the difference here appears to be how well-vetted the pre-1970 data is vetted.

So does "best effort" mean "provide pre-1970 data in the default configuration if we have reason to believe it's accurate, but provide it only in backzone if we don't"?