[tz] Issues with pre-1970 information in TZDB

Michael H Deckers michael.h.deckers at googlemail.com
Fri Sep 24 21:59:27 UTC 2021

    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.

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