Where the Mountain and Pacific time zone line runs

Pacific Time is the continental United States west of 49 CFR 71.9. The line enters Idaho from Montana on the Idaho–Lemhi county border, drops to the Salmon River, and follows that river west to Oregon. Idaho County is split. North of the Salmon is Pacific Time. South of it, including Boise, is Mountain Time (America/Boise, which keeps the same clock as Denver).

In Oregon the line follows Malheur County's north and west sides, then turns east across the south of the county. Most of Malheur is Mountain Time. A southern strip is Pacific, and every other Oregon county is Pacific.

Nevada is Pacific except the City of West Wendover, which the regulation walks around so the city can stay on Mountain Time with Utah. The rest of Elko County, and the rest of Nevada, is Pacific. Arizona is not on this line: the mountain–Pacific boundary there is the Arizona–California border, and Arizona's real quirk is daylight saving time, not the zone.