An easy way to distinguish the line is that it essentially follows the line that divides Washington and Oregon. Idaho counties east of Washington observe Pacific Time, and Idaho counties east of Oregon observe Mountain Time.
For three decades or so in the middle of the twentieth century, Shoshone County had its own time arrangements; it was said to be "on permanent daylight time",[2] so that in the winter the county had the same clock time as Montana and southern Idaho, but in the summer it had the same clock time as Spokane and northern Idaho.
tz database
The tz database version 2024a contains two time zones for Idaho: