[tz] Dealing with Pre-1970 Data

[Lester Caine]

Paul Eggert wrote:
>> >Perhaps*replacing*  the current "LMT" lines for zones with a line
>> >extrapolating the standardized time offsets indefinitely back into
>> >the past might be something worth contemplating.
> If I understand this proposal correctly, it would replace
> tz's current LMT values with values that are less-precise,
> since they'd be rounded to the nearest hour somehow.  For
> (say) Paris, this would result in less-accurate data, since
> it would change Paris's pre-standard-time offset from
> 0:09:21 to zero.  In practice, LMT in Paris before 1891 was,
> I expect, closer to 0:09:21 than to 0, and I don't see how
> changing 0:09:21 to 0 would improve the quality of the
> database entry for Paris.

Skipping the follow on for the moment ...

Paul - MY proposal is that if the database returns LMT then it is a flag that we 
are working with 'pre standard time' dates, and that the actual local time is 
calculated using a longitude value. For the UK we would have Cardif time, Oxford 
time and London time simply by providing the correct location. We would still be 
using the same single zone, and the 'default' would be defined by the location 
used to tag the zone.

Once we move into a time where there is some standardisation, then generally the 
zone gets a LMTZ tag indicating that it is a time applied across a whole zone. 
We may still have to handle the problem of 'train time' and 'government time' 
but this is an historical fact that needs handling anyway and while two zones 
are required it avoids a proliferation of zones in the 'historic' database.

If information turns up that provides more accurate offset to 'local time' than 
the simple location returns then it's a bridge to cross when it happens, but for 
now I think this provides a convenient base for consolidating what material we 
do have available? A simplification may be that the 1970 sub-set just returns 
'LMT' so that there is a valid time, but I get the feeling people would prefer 
what data is available to a cut off set?

-- 
Lester Caine - G8HFL
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