Officially, there are nine US time zones spanning the contiguous United States, the non-contiguous United States, and territories of the United States. The four US time zones within the contiguous United States are, from east to west, the Eastern Time Zone, Central Time Zone, Mountain Time Zone, and the Pacific Time Zone. The Alaska Time Zone includes the U.S. state of Alaska, and the Hawaii-Aleutian Time Zone includes the U.S. state of Hawaii and the Aleutian Islands.
The Eastern Time Zone is the easternmost of all US time zones, and it runs from the Atlantic Coast westward through most of the Ohio Valley. The Central Time Zone runs from the border of the eastern standard time zone westward through the great plains. The mountain standard time zone runs the length of the Rocky Mountains and the states that contain them, and the Pacific Time Zone includes Pacific coastal states as well as the state of Nevada.
The US time zones do not necessarily stop at state borders. For example, South Dakota is divided nearly down the middle between the central standard time zone and the mountain standard time zone. North Dakota's southwest corner is also divided between the two. Indiana is divided between two US time zones, and therefore only part of the state observed daylight saving time, but since 2005, the entire state uses standard time and observes daylight savings time.
As one moves westward through the US time zones, the clock falls back one hour for each time zone. For example, if one passes from the Eastern Time Zone to the Central Time Zone, the time will change at the border from two o'clock p.m. to one o'clock p.m. However, because some sections of the country -- the Hopi Nation in Arizona, for example -- do not observe daylight savings time, this rule does not always hold true.