[tz] State of the tzdb

[Guy Harris]

On Sep 3, 2013, at 1:02 PM, Lester Caine <lester at lsces.co.uk> wrote:

> The I was not talking about any particular statement, but the generality of removing older data from a system that is in active use. Many of the comments relate to winnowing old data and essentially ignoring the historic material. I on the other hand would prefer to be able to retain the pre-1970 material in a format that can be used in exactly the same way as any other time window.

I think Zefram's first usage of the term "winnowing" was when he said:

> If this is a popular idea, I think I should expand a bit on how to deal
> with it.  I alluded earlier to a problem in manual zone selection, where
> the user may be forced to choose between zones that are equivalent for
> eir purposes.  The same approach really addresses both issues, and it's
> worth generalising.
> 
> The essential process is to winnow a set of timezones so that only
> inequivalent zones remain.  Equivalence is in general defined by the user,
> specifically by the user indicating a range of years that is of interest.
> The kind of cutoff discussed so far describes the lower end of the range;
> some applications would also benefit from being able to specify an upper
> end.

The key here is "Equivalence is in general *defined by the user*, specifically by the user indicating a range of years that is of interest."

"The user" might be the ultimate end user or the software packaging using the tzdb.  I.e., the tzdb wouldn't itself force all users to ignore the historic material, but it would *allow* users to do so.

Some or all UN*X systems using the tzdb for localtime() and mktime() might well winnow out all cases where zones differ only prior to 1970, so as to reduce the number of time zones for which somebody configuring the system has to be aware, i.e., not "[forcing] the user to choose between zones that are equivalent for their purposes".

That might, however, surprise some current users of those systems, and applications using the system-supplied version of the tzdb on those systems.

Other applications might winnow out nothing, or might winnow out systems where zones only differ prior to 1900, or some other specified date.