Daylight saving time (DST)—also summer time in several countries, in British English, and European official terminology (see Terminology)—is the practice of advancing clocks so that evenings have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn.

Though mentioned by Benjamin Franklin in 1784, the modern idea of daylight saving was first proposed in 1895 by George Vernon Hudson and it was first implemented during the First World War. Many countries have used it at various times since then.

The practice has been both praised and criticized. Adding daylight to evenings benefits retailing, sports, and other activities that exploit sunlight after working hours, but can cause problems for evening entertainment and other occupations tied to the sun. Although an early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity, modern heating and cooling usage patterns differ greatly, and research about how DST currently affects energy use is limited or contradictory.