What is a time zone?

David Keegel djk at cyber.com.au
Tue Feb 13 23:46:02 UTC 2001

] But for a worldwide standard to make sense, it would have to define exactly
] what a time zone is.  That presents some difficulties.  Different users may
] have varying expectations.  The prevalence of the Anglophone North American
] paradigm has given rise to certain expectations which may be
] self-contradictory.
] Counties in Kentucky
] have opted to switch from Central Time to Eastern Time.  Do we have to call
] them different time zones, or can we use the concept of a time zone that has
] variable boundaries?  Is it within the scope of the standard to describe the
] boundaries, and their evolution through time?

I think common usage outside this group is that a time zone has variable
boundaries, and is something like the set of places which *currently*
observe the same time (although perhaps US EST and CDT might be
considered different time zones).

In this model, counties in Kentucky change time zone, rather than
creating a new time zone, when they switch.  So translating this
mindset to the computer world, if you are in Wayne County you would
manually change your $TZ from US/Central to US/Eastern on 2000-10-29.

Of course, this simplistic model has a fairly obvious problem.  You
can't compute times across a discontinuity like that in any sensible
way, which is why this group has a different concept of what a time
zone is - that it has a history attached and does not have variable
boundaries (in theory). 

Might it reduce this confusion if the TZ group used something like
"time zone history" or "time zone ruleset" instead of just "time zone",
to distinguish it from the epheremal idea of "time zone" in popular

 David Keegel <djk at cyber.com.au>  URL: http://www.cyber.com.au/users/djk/
Cybersource P/L: Unix Systems Administration and TCP/IP network management

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