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The second (symbol: s) (abbreviated s or sec) is the base unit of time in the International System of Units (SI).
In 1956, the second was redefined in terms of a year relative to that epoch. The second was thus defined as the fraction 1 â 31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time. This definition was adopted as part of the International System of Units in 1960. Atomic second