Time is the inteval between two occurences, or duration of events. The usual symbol for time is s and its SI unit is the second, from which larger units are defined like the minute, hour, and day.

Because they do not use the decimal system, and because of the occasional need for a leap-second, the minute, hour, and day are "non-SI" units, but are officially accepted for use with the International System. There are no fixed ratios between seconds (or days) on the one hand and months and years on the other hand -- months and years having significant variations in length. Despite its great social importance, the week is not mentioned even as a "non-SI" unit.

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. In practical terms, the unit of time, the second (s), is 1/86 400 of an average day.

The most common device for measuring time is a watch or clock. More accurate timepieces include the chronometer, the atomic clock, and the solid-state digital timer.The study of time measurement is called horology.

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