[tz] Issues with pre-1970 information in TZDB
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
> 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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