----------------------------Original message----------------------------
Most US counties lie entirely within a time zone.  However,
there are a few counties (see below) which span two time zones.
Can anyone tell me how to determine the precise transition
line (presumably a longitude) which demarcates adjacent time
zones for these anomalous counties?  Any help (including pointers
to appropriate maps) will be appreciated.  I have a copy of
Will Tefft's Map Link time zone map, but it lacks sufficient
detail.
 
State County   FIPS  Time zones
----- ------   ----  ----------
FL    Gulf     12045 -5/-6
ID    Idaho    16049 -8/-7
NE    Cherry   31031 -6/-7
ND    McKenzie 38053 -6/-7
ND    Morton   38059 -7/-6
ND    Sioux    38085 -7/-6
SD    Jones    46075 -6/-7
SD    Stanley  46117 -7/-6
 
P.S. I'll be providing timezones for all US cities in the
Geographic Name Service.