[tz] Dealing with Pre-1970 Data

Paul Eggert eggert at cs.ucla.edu
Sat Aug 31 21:06:54 UTC 2013

Lester Caine wrote:

> having a single repository for ALL available data should be a goal?

Sure, it's a worthy goal, even if pre-1970 timestamps are
currently out of scope for the current database.  We could
collect all the data that we can for an extended database
that contains new zones that differ from existing ones only
for pre-1970 timestamps.  We could then derive the current
database by applying a filter to the extended database,
along the lines that Zefram suggested.  This filtering could
be done automatically and at the source level, so existing
tz source file readers would not need to be changed, and we
wouldn't have to maintain two copies of the database.

As I understand it, Stephen wouldn't oppose the existence of
a filter per se, but is uneasy about having the default
filter being set to 1970.  But I'm afraid the filtering
approach won't work unless we continue to filter at 1970
as we have regularly done in the past.  Too much
existing practice is based in the 1970 cutoff, and (as now
explained in the Theory file) the 1970 cutoff is not really
that arbitrary -- rather, it corresponds roughly with the
advent of computerized timekeeping and of a greater need for
standardized civil time.

> I probably need to point out that as far as I am
> concerned, the UK data is correct back to the creation of
> daylight saving back in 1916

I think you're right, but the UK is quite a special case:
the tz database relies on years of first-class work done by
Joseph Myers and others.  Other countries are not done
nearly as well.  And even the UK entries, which are the best we
have, don't cover all the history of standard time in the
UK, much less pre-standard time.  For more about this,
please see Myers's nice summary

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