[tz] Some thoughts about the way forward

[Guy Harris]

On Sep 24, 2021, at 8:06 PM, Guy Harris via tz <tz at iana.org> wrote:

> Note also that if you want stability of the offset calculation, and somebody else wants errors in pre-1970 timezone information (which may well be present in that information), one of you will have to lose.

...and note that you might have had to avoid 2021b *even if it didn't move anything to backzone*; to quote Paul's announcement:

> Correct many pre-1993 transitions, fixing entries originally
>    derived from Shanks, Whitman, and Mundell.  The fixes include:
>      - Barbados: standard time was introduced in 1911, not 1932; and
> 	DST was observed in 1942-1944
>      - Cook Islands: In 1899 they switched from east to west of GMT,
> 	celebrating Christmas for two days.  They (and Niue) switched
> 	to standard time in 1952, not 1901.
>      - Guyana: corrected LMT for Georgetown; the introduction of
> 	standard time in 1911, not 1915; and corrections to 1975 and
> 	1992 transitions
>      - Kanton: uninhabited before 1937-08-31
>      - Niue: only observed -11:20 from 1952 through 1964, then went to
>        -11 instead of -11:30
>      - Portugal: DST was observed in 1950
>      - Tonga: corrected LMT; the introduction of standard time in 1945,
>        not 1901; and corrections to the transition from +12:20 to +13
>        in 1961, not 1941
>    Additional fixes to entries in the 'backzone' file include:
>      - Enderbury: inhabited only 1860/1885 and 1938-03-06/1942-02-09
>      - The Gambia: 1933 and 1942 transitions
>      - Malawi: several 1911 through 1925 transitions
>      - Sierra Leone: several 1913 through 1941 transitions, and DST
> 	was NOT observed in 1957 through 1962
>    (Thanks to P Chan, Michael Deckers, Alexander Krivenyshev and
>    Alois Treindl.)

Until we reach a fixed point, where all the past tzdb data matches past reality, *if* we ever reach such a fixed point, for past data - and perhaps especially pre-1970 data - "stability" is unlikely to be the tzdb's middle name.