time zone object

Paul Eggert eggert at twinsun.com
Tue Sep 30 08:44:37 UTC 1997

   Date: Mon, 29 Sep 1997 20:25:55 -0700 (PDT)
   From: ncm at cantrip.org (Nathan Myers)

   Many (most?) countries make a practice of announcing changes well in
   advance, and the overwhelming majority of time zone queries will concern
   times in those countries.

Yes, but there's no way to know which countries those are in advance.
If you had asked me two years ago what the expiry time for Sri Lanka
should be, I'd have suggested that it would be a very long time, as
Sri Lanka hadn't futzed with their clocks at all since 1945.  But when
their energy crisis hit critical mass last year, boom!  The clocks got

And surely you're not proposing a database with 0.5-day expiries for
poorer countries' time zone rules, and 12-day expiries for richer
countries', on the theory that the richer countries need the
efficiency more!  Not only would there be political problems with this
rule of thumb, technically speaking it's not very accurate.  The USA,
for example, has historically been much less stable time-zone-wise
than the Dem. Rep. of Congo, which hasn't futzed with its clocks for
nearly a century.

   Hence, an expiry period of fraction of the conventional period for
   such a country (e.g. a month) could radically reduce the TCP
   traffic for zone queries overall.

Other solutions would reduce the IP traffic even further.  For
example, the time zone server could notify the clients of changes
instead of requiring the clients to poll the server.  This could be
done via multicast to cut down on packet transmission even further.

If you're stuck with a protocol in which the client must poll the
server, then probably the best you can do is have the clients poll
once after each reboot or reconnect, and also once a day at a random
time.  One packet-exchange per day should be acceptable overhead for
almost all applications.

By comparison, SNTP sends packets every 64 s to every 1024 s,
depending on configuration.  If this overhead is acceptable, then a
packet every day extra will be in the noise.

   Anyway, the minimum expiry period of most current TZ installations
   is measured in years

I still think you're being too optimistic here.  Yes, most countries
have had relatively stable time zone rules during the last decade or
two.  But come the next war or energy crisis, and the rules will
change again with very little notice.

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