On Sep 24, 2021, at 8:06 PM, Guy Harris via tz <tz@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.