What is a time zone?

[Robert Elz]

    Date:        Fri, 16 Feb 2001 15:47:48 -0500
    From:        "Olson, Arthur David (NCI)" <olsona at dc37a.nci.nih.gov>
    Message-ID:  <4E0F9DDCA724D411B2120008C75DB71DD0F593 at nihexchange2.nih.gov>

I agree with the generality of what you say, but unfortunately ...

  | 	topmost	geographic location to phone system area code
  | 	middle	phone system area code to time zone identifier

that particular layering won't work.   As well as regularly altering
its time zone rules, Australia also just recently altered all its
phone system area codes.   Now we have just 4 area codes for the country.
Anyone following this discussion will know that we have more than
4 timezones (using the tzdata definition of a timezone anyway).
Eg: South Aus, NT, and West Aus are all the same phone area code,
but are three different timezones.   Vic and Tas the same.   NSW and
(I think) Lord Howe Island the same (I'm not certain what area code LHI
uses, but I think it will be the same as NSW).  If Qld ever decides to
do the rational thing and introduce summer time in its south east
corner (one part of Aus, along with north east NSW of course) where
it really would be useful, then that would be another.

Now if you extended the phone system identifier to include the exchange
prefix, things have a better chance at working, but now you have a
nightmare of administration (as those things spring up new almost daily).
And even then, there's no guarantees - phone numbers are allocated to
suit the requirements of the phone system (and to a lesser degree, its
users...) any relationship with timezones is purely accidental.

The relationship really needs to be directly from gegraphical location
(perhaps postal area code, or zip code - those are usually small enough
so they will probably be within a timezone - though even there, they are
assigned to suit the post office, not the sun) to timezone, and from the
same thing to phone area code (if that is needed).

kre