[tz] Proposal: validation text file with releases

Lester Caine lester at lsces.co.uk
Sun Jul 19 08:24:04 UTC 2015

On 19/07/15 01:54, Howard Hinnant wrote:
> Europe/Amsterdam
> Initially:           +00:19:32 standard LMT
> 1834-12-31T23:40:28Z +00:19:32 standard AMT
> 1916-04-30T23:40:28Z +01:19:32 daylight NST

Howard ... Historic material in TZ is still something of a Cinderella
and the fact that sections of it simply become ignored because of the
1970 limit is irritating. The problem I have with the above 'rule set'
is that until there was a common time agreed in 1834, there was not a
previous fixed time for the area over which THAT agreement was made. The
LMT tag does at least say that this time is specific to a point and that
other adjacent towns MAY have different offsets. That the software needs
something to work with is a given, but just as using TZ without the back
file produces incorrect results prior to 1970 so do assumptions like
these. The current demo tzdist claims to be 'unabridged', but in reality
it is only REALLY valid post 1970 and so should flag that limitation.
Paul is totally correct that we will never be able to produce a
perfectly correct historic rules set for the world, but we do know when
the limited set of data we are using started and all I am asking is that
prior to authenticated material, LMT is observed as just that Local Mean
Time as defined by the actual location, and an indication that some
other mechanism maybe needed to calculate local time ... in the UK while
it only applied for a short period of time, towns had their own local
time and that is something that we may well be able to provide via
tzdist while not encumbering TZ with the problems of validating that
data. tzdist needs a geographic lookup system so that anyone can use to
confirm their local timezone, something which geonames currently
provides with a reasonable accuracy, but which is also by no means complete.

Lester Caine - G8HFL
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