[tz] Filling the gaps ...

Lester Caine lester at lsces.co.uk
Tue Dec 5 09:31:08 UTC 2017

The main problem that everybody seems to be skirting around is that the
tz database has NO interest in exactly where someone is located and
there is no remit TO manage that. Even tzdist which was the latest
attempt to plug a major distribution hole has no remit on supplying the
correct data for a particular location! Both are simply managing the set
of rules that anybody can use where ever they are. Importantly allowing
third parts to work out a local time for some elsewhere in the world.

Some other data source is really needed to serve up the CURRENT set of
rules for a location, and that is probably only currently possible using
geonames.org which uses the current timezone_id and it's own simplified
timeZone.txt table! What is missing is any historic (or ongoing) changes
to that information, such as a location that was using one tz rule set
but moved to another. In addition any pre-1970 material can not be

tzdist has the remit and tools to fill one of the holes that tz on it's
own can not and that is historic changes TO the rule sets. My interest
in the tz database came from a major problem with material that had been
normalised to UTC time but there was no record as to what rules had been
used TO normalise the data, so one could not verify if the CURRENT tz
rules were appropriate for establishing the then local times. As Paul
has quite rightly pointed out, much historic data was invented, so
CORRECTING the data when more accurate information is established is
perfectly correct, but one needs to be able to also establish just which
rules have changed and one can then adjust the material that is now
suspect ( - IF APPROPRIATE ;) )

The whole point of tzdist was to sort current day problems, so if a
mobile device with a stored calendar lands in a new location where there
has been an 'unscheduled' change of tz rules that change CAN be
propagated quickly without needing a full tz update but that will only
work if someone is actually running the primary tzdist source. Moving
forward, that primary tzdist source needs to maintain a full historic
record prior to 1970 since it also provides the base from which rules
for historic data normalization can be managed. It allows for the tz
timezone_id's to be used to identify the base rule sets, but allows new
rule sets to be created ... just without any rules on just how THAT is
managed :( Potentially any geonameid could have it's own individual
tzdist dataset and track the history of changes as a location moves from
one political jurisdiction to another and show it's simple solar time as
appropriate. Something that neither tzdist or tz is actually mandated to

Merging geonames.org with openstreetmap.org one gets graphic displays of
the area that timezones CURRENTLY cover, but one can not establish if
the same offsets applied back in the 1960's when DST local times are
well documented. And both databases throw away the historic data to
those mappings along with a whole load of other historic material. While
the vast majority of users probably could not care less, the annoyance
here is that the material existed so it's not as if we are asking for
extra work, just a mechanism that remembers that say in 2014 the Crimea
moved from one set of tz rules to another. Once the mechanism is in
place, then as new historic material comes to light, the historic record
can be corrected and tzdist has the mechanism for that in relation to
timezone rules.

So as a start ... what has happened to tzdist. The full historic record
of tz changes can provide a complete set of data that has been used over
the last 20 years and as new changes are logged in now they can be
pushed to tzdist while also maintaining tz distribution for off-line
population systems.

Lester Caine - G8HFL
Contact - http://lsces.co.uk/wiki/?page=contact
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