[tz] How good is pre-1970 time zone history represented in TZ database?

[Clive D.W. Feather]

Alois Treindl said:
>    tz_count    24'674'767  data records, 100%
>    tz_irange   23'675'636 in time range and region which TZ covers
>    correctly, 96%
>    tz_good        736'769 in time range and region where TZ is unreliable,
>    but correct, 3%
>    tz_bad         262'362 in time range and region where TZ database gives
>    false result. 1%
> 
> Can you please explain how you've done this analysis. In particular, how do
> you know that TZ is "unreliable, but correct" or gives a bad result?

[...]

Okay, to paraphrase what you said:

* "irange" means that the zone that covers the relevant place now covered
it at the relevant time. For example, the Europe/Berlin rules applied at
the relevant time, even if that was pre-1970.

* "good" means that the relevant place had different time rules before 1970
to the zone that now applies, so is in a different zone if you include
pre-1970 rules, but the two zones had the same offset at the relevant time.
For example, the Europe/Berlin rules disagreed with (hypothetical)
Europe/Kiel rules but gave the same result at the relevant time.

* "bad" means that according to the information you hold the relevant place
had a different offset at the relevant time to the zone that now covers it.
For example, Europe/Berlin and Europe/Kiel have different offsets at the
relevant time.

Have I understood correctly?

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