Mtn/Cntrl border at Fort Pierre South Dakota (also Pac/Mtn border at Winterhaven California)

Steve Jones stevejones at
Thu Apr 7 04:45:10 UTC 2011

At 10:03 3/23/2011, Steve Jones wrote:
>The current Mountain/Central boundary in the most recent official US 
>DOT description (49CFR71.7,, has the border 
>running right through Fort Pierre South Dakota.

A follow up:

In the above I made a major error by failing to recognize the 
significance of this part of 49CFR71.7:

>(g) Points on boundary line. All municipalities located upon the 
>zone boundary line described in this section are in the mountain 
>standard time zone, except Murdo, S. Dak., which is in the central 
>standard time zone.

So all of the municipality of Fort Pierre is indeed within the 
Mountain Time zone.

Today I released v1.05, which includes my 
interpretation of the Fort Pierre anomaly, as well as a similar 
situation in Winterhaven California, which, in respect to proximity 
to Yuma AZ, observes what it calls "Arizona Time" (Mountain Standard 
Time) year round.

Here is what the site says about these changes:

>SD: The city of Fort Pierre (legally in the Mountain time zone) and 
>a large radius on the west side of the Mountain/Central border 
>observes Central time. Research determined that the distance this 
>observance extends to the west is a matter of disagreement among the 
>locals. After some effort to get a definitive answer I gave up and 
>arbitrarily selected 20 miles, though I heard credible opinions that 
>it stretched out as much as 50 miles. There are no services and very 
>few residents in the western part of Stanley county, so the point is 
>somewhat moot.
>CA: Winterhaven California, in deference to its proximity to Yuma 
>AZ, observes Mountain Standard Time year round. This means that 
>(like AZ) Winterhaven does not observe Daylight Saving time. So this 
>puts Winterhaven ahead of California by one hour (in sync with 
>Arizona) during Standard time - in the Fall and Winter. And in sync 
>with both CA and AZ during Spring and Summer (Daylight Saving time). 
>The dividing line of this observed boundary is locally considered to 
>be the "All American Canal".

I hope this information is of use to some here.


Steve Jones 
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