You should set clocks ahead at 2 a.m. on the second Sunday in March in most parts of the United States. However, Hawaii, Arizona and several U.S. territories do not participate in Daylight Saving Time.
Daylight Saving Time shifts one hour of daylight from the beginning of the day to the end of the day between the second Sunday in March and the first Sunday in November. The United States implemented Daylight Saving Time to save energy, and a 1975 study suggests that the United States initially saved approximately 1 percent in energy costs by decreasing evening lighting and home appliance use. As more individuals install home air conditioners, however, shifting evening activity from the workplace to home on hot summer evenings appears to increase energy costs.