[tz] changes to the TZ database over versions

[Alan Barrett]

On Sun, 01 Jun 2014, Joris Van den Bogaert wrote:
> Looking at the git log, it looks like most changes are typos and 
> adjustments to data far away in the past, like the early 1900's 
> or Big Bang :) changes and sometimes the very near future, like 
> Egypt and Turkey.
> I didn't find any substantial changes that were made in the near 
> past.

There are occasionally changes that affect the recent past, such 
as when governments make changes with very short notice.  It is 
sometimes the case that the change has already taken effect in the 
real world before the tz database is updated, or at least before 
the new version is published and widely used.

> Hypothetically, let's say Catalonia becomes a separate state 
> on 1/1/2015 and decides to have a timezone UTC+01:30 instead 
> of CET.  Do you guys then decide to create a new timezone 
> Europe/Barcelona?

In that hypothetical situation, a new timezone would be created, 
and named after the largest city or population centre in the 
affected area, which could well be Barcelona.

> What (should) happens when converting the datetime 1/1/2014 with 
> "Europe/Barcelona", a timezone that didn't exist at that time?

Any hypothetical new Europe/Barcelona zone would attempt to 
report the time as it was in the city of Barcelona at least as 
far into the past as 1 Jan 1970 (or perhaps farther, if good 
records exist), and as far into the future as can reasonably be 
predicted from legislation and other sources.  So 1 Jan 2014 in a 
hypothetical new Europe/Barcelone zone would be identical to 1 Jan 
2014 in the existing Europe/Madrid zone.

--apb (Alan Barrett)