[tz] Time zone selection

Lester Caine lester at lsces.co.uk
Fri Apr 22 20:07:37 UTC 2016

On 22/04/16 19:52, Brian Inglis wrote:
> On 2016-04-22 11:44, Lester Caine wrote:
>> One of the complaints has been that there are too many sets of timezone
>> rules, but combining several zones that NOW have the same set of rules
>> misses the point that there IS a lot of historic material and looking
>> back at even just as far as the 2nd world war requires subtle
>> differences. It would seem that the historic changes in Russian
>> locations are also now getting accurate amendments to the rules, and
>> simply hiding that information in 'backzone' needs to be clearly
>> displayed when people using calendars that have 'simplified things' by
>> stripping anything prior to 1970. Something substantial to prevent users
>> from being given any offsets when the data is not available.
> Return -1 for all dates prior to the epoch if zone1970.tab was used to
> build?

That would at least warn that a time zone offset is not reliably available.

The one thing that we were trying to get people to understand on the
tzdist list is that a key element of this is not just the offset, but
that you know just which set of rules was used to create an historic
normalized calendar. A lot of historic material has had to be completely
reworked simply because we do not KNOW which tz data used at the time.
The example of a US location changing timezone currently means that any
normalized data on the computers in the 'affected' area needs to retain
the rule set for the previous period and recalculate the diary for the
coming period. So you need a rule set which contains ALL the correct
data. Otherwise you have to have a diary that knows which rule set was
used for which part of the data ... Yesterday is history just as much as
pre-1970 ...

Lester Caine - G8HFL
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