Geographical boundaries for the continental US tz zones

Gwillim Law RLAW at
Thu May 31 23:39:39 UTC 2007

My page at has a good deal of detail about
time zone changes over the years in the United States.  I think you'll be
able to find the answers to some of your questions there.  I've answered #2
because I found the information right away.

Most of my description of time zone boundaries comes from published maps,
usually state road maps from Rand McNally, the AAA, Gousha, etc.  They're
pretty reliable, but they can't be considered primary sources.  In the list
of changes following the table on my webpage, the changes with exact dates
often come from primary sources (DOT docket items) or widely disseminated
news reports.  Changes since about 1988 can usually be confirmed by
examining the tz mailing list archives.

--   Gwillim Law

----- Original Message ----- 
From: "Eric Muller" <emuller at>

> 2. The description of America/Denver includes "far eastern Oregon", and
> the description of America/Boise includes "eastern Oregon". Comparing
> with timeznp020, I suspect that America/Denver should not include "far
> eastern Oregon". (and if timeznp020 is right, "eastern Oregon" could be
> better described as "most of Malheur County, Oregon").

Eastern Oregon is definitely equivalent to "most of Malheur County",
although if the road maps are correct, the exact boundaries underwent a
small change around 1970.  The change affected only sparsely populated
areas.  As the tz comments say, southern Idaho and eastern Oregon delayed
going on DST for four weeks in 1974.  Eastern Oregon belongs in
America/Boise, because it remained in synch with Boise rather than Denver
during that period.

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