Rationale An ancient water clock that lets hour lengths vary with season A minority of the world's population uses DST-Asia, Africa, and Latin America and the Caribbean generally do not.

The United States observes it, except for the states of Hawaii and Arizona (within the latter, however, the Navajo Nation observes it, conforming to federal practice). Some countries observe it only in some regions: for example, parts of Australia observe it, while other parts do not conversely, it is not observed at some places at high latitudes because there are wide variations in sunrise and sunset times and a one-hour shift would relatively not make much difference at all. DST is generally not observed near the Equator, where sunrise and sunset times do not vary enough to justify it. Many countries have used DST at various times since then, particularly since the 1970s energy crisis. Starting on 30 April 1916, the German Empire and Austria-Hungary each organized the first nationwide implementation in their jurisdictions. In 1908, Port Arthur in Ontario, Canada (today merged into Thunder Bay), started using DST. After some serious consideration, it was not implemented.

In 1907, British resident William Willett presented the idea as a way to save energy. In 1895, New Zealand entomologist and astronomer George Hudson proposed the idea of changing clocks by two hours every spring to the Wellington Philosophical Society. In a satirical letter to the editor of The Journal of Paris, Franklin suggested that waking up earlier in the summer would economize on candle usage and calculated considerable savings. The idea of aligning waking hours to daylight hours to conserve candles was first conceptualized in 1784 by American polymath Benjamin Franklin. As a result, there is one 23-hour day in early spring and one 25-hour day in the middle of autumn. The typical implementation of DST is to set clocks forward by one hour in either the late winter or spring (' spring forward'), and to set clocks back by one hour in the fall (North American English) (' fall back') or autumn (UK English) to return to standard time. Daylight saving time ( DST), also referred to as daylight savings time, daylight time (United States, Canada, and Australia), or summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typically by one hour) during warmer months so that darkness falls at a later clock time.

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