Daylight saving time (DST) or summer time is the practice of advancing clocks during summer months by one hour so that in the evening daylight is experienced an hour longer, while sacrificing normal sunrise times. Typically, regions with summer time adjust clocks forward one hour close to the start of spring and adjust them backward in the autumn to standard time.

New Zealander George Hudson proposed the modern idea of daylight saving 1895. Germany and Austria-Hungary organized the first nationwide implementation, starting on 30 April 1916. Many countries have used it at various times since then, particularly since the energy crisis of the 1970s.

The practice has received both advocacy and criticism. Putting clocks forward benefits retailing, sports, and other activities that exploit sunlight after working hours, but can cause problems for evening entertainment and for other activities tied to sunlight, such as farming. Although some early proponents of DST aimed to reduce evening use of incandescent lighting, which used to be a primary use of electricity,  modern heating and cooling usage patterns differ greatly and research about how DST affects energy use is limited or contradictory.