[tz] Adding verified historic details

[Lester Caine]

Guy Harris wrote:
> On Sep 4, 2013, at 2:15 PM, Lester Caine <lester at lsces.co.uk> wrote:
>> If you want a 1970+ only database then it should be advertised as that,
> As far as I know, he wants the ability for packagers of the database to create, from the tzdb, 1970+-only databases if they deem pre-1970 data not worth keeping, not to permanently expunge pre-1970 differences from the tzdb itself.
Then it's up to the distributions to do that - in a manor that is compatible wth 
the rest of their infrastructure ... such as not having a pre-1970 calendar.

>> and we can create a second one which has all of time
> If we're not going to be adding new tzids by splitting zones that differ only pre-1970, and there are currently tzids that cover a single region that has different pre-1970 behavior in different parts of the region, we're not going to have "a second one that has all of time" accurately.
A proposal has been put forward to prevent the creation of new timezones just 
for different locations prior to standard time being adopted. Pre-standard time 
LMT is assumed and calculated independent of TZ.

If there is documentary evidence of a different time pattern such has been added 
for the Isle of Man and is about to appear for the channel islands then it 
should be allowed not blocked.

Where different locations adopt standard time at different documented times then 
the source file format needs to be change to accommodate that. I proposed that 
the location and date is recorded and then links to the following timezone for 
the rest of the data. The Isle of man is a perfect example of this since all it 
differs in is the date of the start of using GMT.

>> any distribution and API should be using the full range, just as their calendar should be accurate for historic dates.
> Any distribution and API has the option to use whatever they choose; some particular UN*X might well themselves choose to merge tzids that differ only pre-1970.  That's not for us to dictate.  (Heck, as somebody noted, QNX has an *unsigned* seconds-since-the-Epoch time_t, so it's literally *impossible* to specify a pre-January 1, 1970, 00:00:00 UTC date as a time_t in QNX, so anything in QNX that use the tzdb to convert time_t's will never look at the pre-1970 data in the tzdb.)
So why are you dictating that pre 1970 data will be stripped?
"That's not for us to dictate."

-- 
Lester Caine - G8HFL
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