[tz] Dealing with Pre-1970 Data

Paul Eggert eggert at cs.ucla.edu
Mon Sep 2 07:37:29 UTC 2013

Guy Harris wrote:

> you're probably *still* going to get multiple cities within the same zone
> even if you change the cutoff to 1911 or even 1891:

Absolutely.  The further back you move the cutoff,
the more zone splits you get, but if we're only talking
about standard time there is a limit of the total number
of zones.  I'm guessing it's in the thousands, though
it's just a guess.

> "why should certain cities be blessed by the tzdb by virtue of having their
> particular local solar mean time in the database"?

The answer to that is the same as the answer to the question
"why should certain cities be blessed by having their names
in the tz database at all"?  They're the largest
cities in their respective zones, that's all.

> maybe we should just have localtime() return NULL for times
> prior to the adoption of standard time

That might make sense.  And it would conform to POSIX
in practice, since standard time was introduced everywhere
before 1970, and POSIX doesn't define behavior before 1970.
Still, it would be a little weird for localtime() to stop
working for dates before 1847 when TZ='Europe/London',
simply because the time was different in London
than it was in (say) Oxford.

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