[[image:DaylightSaving-World-Subdivisions.png|300px|thumb| Although not used by most of the world's people, daylight saving time is common in high latitudes. legend: DST used legend: DST no longer used legend: DST never used ]] Daylight saving time (DST; also, summer time in British English; see Terminology]]) is the convention of advancing clocks so that afternoons 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. Modern DST was first proposed in 1907 by the English builder [[William Willett. Many countries have used it since then; details vary by location and change occasionally.

The practice is controversial. Adding daylight to afternoons benefits retailing, sports, and other activities that exploit sunlight after working hours, but causes problems for farming, entertainment and other occupations tied to the sun. Extra afternoon daylight reduces traffic fatalities; its effect on health and crime is less clear. 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 greatly differ and research about how DST currently affects energy use is limited and contradictory.

DST's clock shifts can serve as fire safety reminders, but they complicate timekeeping and can disrupt meetings, travel, billing, recordkeeping, medical devices, and heavy equipment. Many computer-based systems can adjust their clocks automatically, but this can be limited and error-prone, particularly when DST rules change.

Origin

[[image:Clepsydra-Diagram-Fancy.jpeg|thumb| In this ancient water clock, a series of gears rotated a cylinder to display hour lengths appropriate for each day's date.]] Although not punctual in the modern sense, ancient civilizations adjusted daily schedules to the sun more flexibly than modern DST does, often dividing daylight into twelve equal hours regardless of day length, so that each daylight hour was longer during summer. For example, Roman water clocks had different scales for different months of the year: at Rome's latitude the third hour from sunrise, hora tertia, started by modern standards at 09:02 solar time and lasted 44 minutes at the winter solstice, but at the summer solstice it started at 06:58 and lasted 75 minutes. After ancient times, equal-length civil hours eventually supplanted unequal, so civil time no longer varies by season. Unequal hours are still used in a few traditional settings, such as some Mount Athos monasteries.