[tz] changes to the TZ database over versions

[Paul Eggert]

Joris Van den Bogaert wrote:

> 1) I noticed that certain timezone ids are deleted in newer versions: tzdata2014a contains
> America/Shiprock, newer versions don't.

No, America/Shiprock is still present in the latest tz release.  It's in 
the 'backward' file.  If you're concerned about backward compatibility, 
you should use the 'backward' file.

> 2) Do rules from the past ever change?

Yes, it happens all the time as we find out more about the past (or, in 
some cases, find out that what we thought we knew was bogus).  A 
proposed change of that sort for Moscow in 1921 was published today, for 
example; see:

https://github.com/eggert/tz/commit/8d558674ce15736f4db98332e9d1e86b1555c340

> would one then recommend saving the TZ database version

One might, if one knew that one was using exactly a particular TZ 
database version.  But that's often not the case; if someone else 
installed your database, they may have applied their own updates, e.g., 
point updates for pressing changes.  And depending on your 
configuration, perhaps the TZ database might be updated during your 
computation.

So it might be wise for you to record both UTC and local time, instead 
of trying to rely on tz version.