Daylight Saving Time (DST) is a period of the year between spring and fall when clocks in most parts of the United States are set one hour ahead of standard time. The beginning and ending dates and times—the second Sunday in March at two o'clock ante meridian and the first Sunday in November at two o'clock ante meridian—are set in statute. Congressional interest in the potential benefits and costs of DST has resulted in changes to DST observance since it was first adopted in the United States. Congress may consider whether to make additional changes to DST observance or standard time. Congress may also consider whether additional examination of potential benefits and costs of DST observance would be informative. This report presents a selected history of DST in the United States and includes a summary of potential effects, selected state actions related to DST or standard time, and federal legislation that has been introduced related to DST in the 115 th Congress. The development of railroads led to the creation of a standard time in the United States. To reduce the confusion resulting from locally established "sun times" at railroad terminals, U.S. and Canadian railroads adopted four standard time zones in 1883. The four established times zones were the Eastern, Central, Mountain, and Pacific. Adoption of time zones at the local level was influenced by the time zones adopted by the railroads. In 1918, Congress passed an act to provide standard time for the United States (also known as the Calder Act and the Standard Time Act of 1918). The act established five standard time zones; the fifth time zone was established to include Alaska. It authorized the Interstate Commerce Commission (ICC) to fix the time zone boundaries within the United States and to change them as necessary. The act also set a summer DST to begin on the last Sunday of March and conclude on the last Sunday in October. The adoption of DST in the United States was preceded by adoption of DST in Europe during World War I. After World War I, Congress abolished summer DST at the federal level, although it remained a local option with some states continuing to observe it. In 1961, the Interstate Commerce Commission found the lack of nationwide consistency in time observance—including the observance and length of DST—confusing enough to recommend that Congress reexamine "the entire field of standard time." This reexamination resulted in the Uniform Time Act of 1966 (P.L. 89-387). The act mandated standard time within the established time zones and provided for advanced time: clocks would be advanced one hour beginning at 2:00 a.m. on the last Sunday in April and turned back one hour at 2:00 a.m. on the last Sunday in October. States were allowed to exempt themselves from DST as long as the entire state did so. If a state chose to observe DST, the time changes were required to begin and end on the established dates. The ICC was authorized with implementing the act until the authorities were transferred to the Department of Transportation (DOT), which occurred in October 1966 through the Department of Transportation Act (P.L. 89-670). In 1972, the Uniform Time Act was amended (P.L. 92-267) to allow states that were split between time zones to exempt either the entire state or that part of the state lying within a different time zone. The following states and territories do not observe DST: American Samoa, most of Arizona, Guam, Hawaii, the Northern Mariana Islands, Puerto Rico, and the Virgin Islands. Year-round DST has been implemented twice on a temporary basis in the United States. In 1942, Congress enacted a law to institute year-round DST, which became known as "War Time." The law advanced the standard time for each time zone by one hour to be in effect until six months after the war or earlier if Congress were to require in a subsequent concurrent resolution. War Time concluded on the last Sunday in September 1945. During the 1973 oil embargo by the Organization of the Petroleum Exporting Countries (OPEC), Congress enacted the Emergency Daylight Saving Time Energy Conservation Act of 1973 ( P.L. 93-182 ), which established a trial period of year-round DST, from January 6, 1974, to April 27, 1975. The act was amended in October 1974 by P.L. 93-434 , which required a return to standard time for the period beginning October 27, 1974, and ending February 23, 1975, when DST resumed. When the trial period ended in 1975, the country returned to observing summer DST (with the aforementioned exceptions). Congress has made additional changes to the Uniform Time Act. In 1986, Congress enacted P.L. 99-359 , which changed the beginning of DST to the first Sunday in April. In 2005, Congress enacted the Energy Policy Act of 2005 ( P.L. 109-58 ). Section 110 of this act amended the Uniform Time Act, by further changing DST to begin the second Sunday in March and end the first Sunday in November; this DST period remains in effect today. The act required the Secretary of the Department of Energy (DOE) to report to Congress on the impact of extended DST on energy consumption in the United States (see section on " Energy Savings "). Other legislative changes addressed time zones. P.L. 106-564 amends the Calder Act to increase from eight to nine the number of standard time zones in the territory of the United States, creating Chamorro standard time, which includes Guam and the Commonwealth of the Northern Mariana Islands . It also amended the Uniform Time Act of 1966 to include Guam and the Northern Mariana Islands in the definition of "State." In 2007, the 110 th Congress enacted the America COMPETES Act ( P.L. 110-69 ). Section 3013 of this act made changes to the Uniform Time Act regarding Standard Time in the United States, including a change to Idaho's time zone. Support and opposition to changes in DST observance often focus on its potential effects. Those in favor of extending DST observance reportedly argue that the extended daylight in the evening promotes economic activity, including tourism. Those against extending DST observance reportedly argue that losing an hour of daylight in the morning could negatively affect golf course operations, ski resorts, and schoolchildren's morning commutes. A number of studies have been conducted on DST's effects on safety, energy savings, and health. A selection of these—including studies mandated by Congress—is discussed in this section. This is not a comprehensive review of the literature. Congress has required several agencies to evaluate the effects of DST. DOT provided an interim evaluation and final report on the operation and effects of DST in 1974 and 1975. The interim report's findings were inconclusive, stating that the observed effects were "so small that they could not in general be reliably separated from effects of other changes occurring at the time." The report concluded that, "there is no unambiguous direct evidence that [the measurable effects of year-round daylight saving time] were either beneficial or harmful." DOT reported that "modest overall benefits might be realized by a shift from the historic six-month DST (May through October) [to year-round DST] in areas of energy conservation, overall traffic safety, and reduced violent crime." DOT also reported that these potential benefits would be minimal and difficult to distinguish from seasonal variations and fluctuations in energy prices. After DOT submitted the report to Congress in 1975, Congress directed the National Bureau of Standards (NBS) to conduct a technical evaluation of the DOT study and to deliver a report to Congress. In its report, NBS found no significant energy savings or differences in traffic fatalities. Although statistically significant evidence was found of increased fatalities among school-age children in the mornings during the four-month period January-April 1974 as compared with the same period (non-DST) of 1973, NBS stated that This increase cannot be simply interpreted as 'DST effect,' in view of the many other factors that influence traffic fatalities. When the months of January through April are considered individually, there is no statistically significant difference between 1973 and 1974 with respect to school-age children fatal accidents in the morning for the months of March and April. Other studies have found that the short-term effect of DST on crashes is not statistically significant. Some reviews of the scientific literature have found that the long-term effects of DST suggest a positive effect. One such report noted, however, that the results "may be attributable to factors other than light." As part of P.L. 109-58 , Congress directed DOE to evaluate potential energy savings that may result from extending DST. In 2008, DOE submitted a report to Congress on the effects of extended DST on U.S. energy consumption. DOE found that the total electricity savings associated with extended DST corresponded to 0.03% of electricity consumption in 2007. In terms of primary energy consumption, this represents approximately 0.02% of total U.S. energy consumption in 2007. Regionally, areas in the northern United States exhibited larger electricity savings than areas in the southern United States. DOE reported that there was insufficient statistical evidence to indicate whether extended DST "had any measurable impact on motor gasoline consumption for passenger vehicles or traffic volume in 2007." Literature reviews have reported similar savings as reported by DOE, although at least one empirical study found that DST increases electricity demand. Aries and Newsham (2008) concluded that "existing knowledge about how DST affects electricity use is limited, incomplete, or contradictory." Havranek et al. (2018) conducted a literature review and found that "electricity savings are larger for countries [or areas] farther away from the equator, while subtropical regions consume more electricity because of DST."