[tz] zone.tab / zone1970.tab inconsistency for TF/UM/VN

[Neil Fuller]

Ok. Thanks for explaining. I think I get it, and the summary of my
understanding is that:

The zone.tab will have fewer zones than zone1970.tab when to represent the
pre-1970 / zone.tab rules it would require the synthesis of a new zone ID
purely to represent the < 1970 situation.

i.e. in the case of TF and Indian/Reunion, Indian/Reunion can be used for
TF after 1970 but it would not be correct before 1970, and there is no
existing candidate that would be.

Thanks!

Neil.

On 31 March 2017 at 21:03, Paul Eggert <eggert at cs.ucla.edu> wrote:

> Neil Fuller via tz wrote:
>
> I expect the exact zone IDs to be different, but there are only a few cases
>> where countries differ in the number of referenced zone IDs.
>>
>
> These discrepancies come up because zone.tab tries to be compatible with
> the old tz rules, whereas the rest of the data follow the new rules (and
> zone1970.tab reflects the new rules). The old rules weren't as well
> documented, I'm afraid, so this is a bit of a murky area.
>
> I realize that zone.tab is intended for backward compatibility but it looks
>> like it is being well maintained.
>>
>
> Yes, zone.tab is still quite commonly used. How long this will continue to
> be true is anybody's guess. Perhaps until the next political crisis....
>
>
> zone.tab:
>>
>> TF -492110+0701303 Indian/Kerguelen
>>
>> zone1970.tab:
>>
>> RE,TF -2052+05528 Indian/Reunion Réunion, Crozet, Scattered Islands
>> TF -492110+0701303 Indian/Kerguelen Kerguelen, St Paul Island, Amsterdam
>> Island
>>
>
> In theory according to the old rules, there should be two zones for TF:
> the existing one and another zone for Crozet and the Scattered Islands.
> The new rules say that we don't need the latter zone, as it's identical to
> Indian/Reunion since 1970. (And thank goodness! hardly anyone lives on
> Crozet etc., and info about civil time there is hard to come by.)
>
> zone.tab approximates the old rules with the new data as best it can. It
> doesn't use Indian/Reunion under TF as I expect consumers of zone.tab
> expect zone.tab to be a 1-1 relation.
>
>
> zone.tab:
>>
>> UM +2813-17722 Pacific/Midway Midway Islands
>> UM +1917+16637 Pacific/Wake Wake Island
>>
>> zone1970.tab:
>>
>> AS,UM -1416-17042 Pacific/Pago_Pago Samoa, Midway
>> UM +1917+16637 Pacific/Wake Wake Island
>> US,UM +211825-1575130 Pacific/Honolulu Hawaii
>>
>
> The Pacific/Honolulu link is to UM as Indian/Reunion is to TF. That is, it
> stands for minor islands that would need a separate zone under the old
> rules, but not for the new ones. Johnston Island comes to mind.
>
> Pacific/Midway is an old-fashioned alias for Pacific/Pago_Pago, so
> zone.tab uses it to preserve the illusion that it's 1-1. It can't do this
> for TF and Indian/Reunion as the latter has no aliases. We don't create
> aliases merely to do minor cleanups of zone.tab for backward-compatibility
> purposes.
>
>
> zone.tab:
>>
>> VN +1045+10640 Asia/Ho_Chi_Minh
>>
>> zone1970.tab:
>>
>> TH,KH,LA,VN +1345+10031 Asia/Bangkok Indochina (most areas)
>> VN +1045+10640 Asia/Ho_Chi_Minh Vietnam (south)
>>
>
> Asia/Bangkok is to VN as Indian/Reunion is to TF. It stands for part of
> Vietnam that would need a separate zone under the old rules, but not the
> new ones. The region in question is the old North Vietnam. It has no
> aliases, so zone.tab doesn't use it under VN.
>



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