[tz] Issues with pre-1970 information in TZDB

[Paul Eggert]

On 9/22/21 15:23, Howard Hinnant via tz wrote:

> Before the change the tzdb does a “best effort” on pre-1970 dates.

No, quite the reverse. There are many places in 2021a where we have some 
data (often unreliable) about pre-1970 timestamps, and we put that data 
into 'backzone' instead of doing a "best effort" on pre-1970 dates in 
the default database. Examples include Africa/Addis_Ababa, 
Africa/Freetown, and Asia/Harbin. I expect that there are more Zones in 
2021a where this sort of thing happens, than Zones in the changes being 
discussed.

There's a lot of precedent for the recent changes, and a lot of 
experience for the fact that the fallout from this kind of change is 
negligible in practice.