49 CFR 71.9 - Boundary line between mountain and Pacific zones.
(a) Montana-Idaho-Oregon. From the junction of the Idaho-Montana boundary with the boundary between the United States and Canada southerly along the Idaho-Montana boundary to the boundary line between Idaho County, Idaho, and Lemhi County, Idaho; thence southwesterly along the boundary line between those two counties to the main channel of the Salmon River; thence westerly along the main channel of the Salmon River to the Idaho-Oregon boundary; thence southerly along that boundary to the boundary line between Baker County, Oreg., and Malheur County, Oreg.; thence westerly along the north line of Malheur County to the northwest corner of Malheur County; thence southerly along the west line of Malheur County to the southwest corner of T. 35 S., R. 37 E.; thence east to the Idaho-Oregon boundary; thence south along that boundary to the southwest corner of the State of Idaho; thence easterly along the Idaho-Nevada boundary to the northeast corner of the State of Nevada.
(b) Utah-Nevada-Arizona-California. From the northeast corner of the State of Nevada southerly along the Utah-Nevada boundary to the junction with the northern border of the City of West Wendover, Nevada. Then westward along the northern, western, and southern boundaries of the City of West Wendover back to the Utah-Nevada boundary. Then southerly along the Utah-Nevada boundary, the Nevada-Arizona boundary, and the Arizona-California boundary to the boundary between the United States and Mexico.
(d) Municipalities on boundary line. All municipalities located upon the zone boundary line described in this section are in the mountain standard time zone.