What is a time zone?

[Olson, Arthur David (NCI)]

The existing public-domain time zone stuff lets you know how to represent
local time if you provide the identity of the set of rules that apply.
Recent mail has dealt with the matter of handling things such as geographic
locations or phone system area codes.

My sense is that layered software is the way to go. The bottom level turns a
time-zone identifier 
into a representation of the rules that apply. Built on top of this are
packages that take different forms of input (phone system area code,
geographic location) and deliver up time-zone identifiers. The existing
time-zone stuff is a bottom level. And there might be multiple higher
levels--for example:
	topmost	geographic location to phone system area code
	middle	phone system area code to time zone identifier
	bottom	time-zone identifier to rules that apply
The organization of the higher levels would, of course, be immaterial to
what goes on at the bottom level.

A consideration in determining the time-zone identifiers to use in such a
setup is the independent usability of the bottom level. The existing time
zone stuff uses identifiers of the form big-geographic-region/city; this
allows some meaningful applications (such as setting the local time to be
used on a computer) without recourse to a higher level (proof by existence).
There are other considerations as well (technological, political,
technological, astrological...). And determining what to use as a time-zone
identifier ought not be done in a vacuum: if the same identifier can be used
for other purposes, we've simplified our lives.