Why does the U.S. Use Daylight Saving Time? Daylight Saving Time has been legally enforced in the U.S. on-and-off since 1918, when congress passed the Standard Time Act. The law set the boundaries for the various time zones in the U.S. The purpose of Daylight Saving Time is to maximize use of sunlight hours in the Northern hemisphere during the summer months, when the sun typically rises earlier. [time-brightcove not-tgx=”true”] Read More: Is the U.S.