[tz] models for timezones

Guy Harris guy at alum.mit.edu
Mon Feb 12 04:18:39 UTC 2018

On Feb 11, 2018, at 7:45 PM, Steve Summit <scs at eskimo.com> wrote:

> In that old model, a time zone has one offset from Greenwich/UTC,
> and maybe a second offset on top of that to realize "daylight
> saving time", and one set of rules for deciding when to
> transition to and from DST.  (Once upon a time, that set of rules
> might have been limited to the pre-1987 rules for the United
> States, and it might have been hard-compiled into C code.  With
> the advent of the Posix TZ environment variable definition it's
> at least a little bit more flexible.)

Which is why I prefer calling the things identified by tzdb identifiers "tzdb regions" rather than "time zones" - a tzdb region may, over its history, be in more than one of what people (at least in the US and, I suspect, Canada) think of as "time zones", and there may be more than one tzdb region in a given "time zone".

And theory.html starts out saying

	To represent this data, the world is partitioned into regions whose clocks all agree about timestamps that occur after the somewhat-arbitrary cutoff point of the POSIX Epoch (1970-01-01 00:00:00 UTC).

although it later refers to those regions as "time zones".

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