On 2021-09-24 13:42, Brooks Harris via tz asked:
Why is Asia/Tokyo retained while Europe/Oslo is moved?
Let me try to explain. Europe/Oslo and Europe/Berlin agree since 1966, and tzdb only wants to describe local time scales since 1970, so one of them suffices. The tzdb rule for that case mandates that the larger city is taken; the other city (Oslo) is moved to backzone, where all the currently unnecessary timezone data are kept. As for Asia/Tokyo, when it is found that its local time agrees with that of Pacific/Palau since 1952, then Palau will be moved to backzone. While the typical interfaces require that local times extend before 1970, the exact values for the far past are usually not so important except for a few user groups: astronomers and astrologers need exact data for the past, while data bases and similar systems need stable data for the past. When a user specifies Europe/Oslo, she gets the data of Europe/Berlin unless the system she uses was built with backzone included (which is rare). This leads to surprises: when Berlin switches to permanent summer time some time in the future, then Europe/Oslo must be released from backzone, and the pre-1970 data for Oslo will change for no reason obvious to the user. Therefore, for better stability backzone should be included (at least the part with post-1970 duplicates), while smaller data size is achieved without it. Michael Deckers.