[tz] Issues with pre-1970 information in TZDB

Brooks Harris brooks at edlmax.com
Fri Sep 24 23:03:06 UTC 2021

On 2021-09-24 5:59 PM, Michael H Deckers wrote:
>    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.
Hi Michael, thanks.

So smaller data size is the driving motivator?

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