Commission de décimalisation du temps was created by the French Bureau of Longitude, with the mathematician Henri Poincaré as secretary. The commission proposed making the standard hour the base unit of metric time, but the proposal did not gain acceptance and was eventually abandoned. When the modern SI system was defined at the 10th General Conference on Weights and Measures (CGPM) in 1954, the ephemeris second (1/86400 of a mean solar day) was made one of the system's base units. Because the Earth's rotation is slowly decelerating at an irregular rate and was thus unsuitable as a reference point for precise measurements, the SI second was later redefined more precisely as 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 caesium-133 atom. The international standard atomic clocks use caesium-133 measurements as their main benchmark. In computing In computing, at least internally, metric time gained widespread use for ease of computation. Unix time gives date and time as the number of seconds since January 1, 1970, and Microsoft's NTFS FILETIME as multiples of 100 ns since January 1, 1601. VAX/VMS uses the number of 100 ns since November 17, 1858, and RISC OS the number of centiseconds since January 1, 1900. Microsoft Excel uses number of days (with decimals, floating point) since January 1, 1900. All these systems present time for the user using traditional units. None of these systems is strictly linear, as they each have discontinuities at leap seconds. Prefixes Metric prefixes for subdivisions of a second are commonly used in science and technology. Milliseconds and microseconds are particularly common. Prefixes for multiples of a second are rarely used: See also List of unusual units of measurement#Time, under which prefixed multiples of the second are included Soviet calendar References External links Metric unit of time (second) Official text of SI brochure from International Bureau of Weights and Measures Metric Time? University of Illinois Physics Department International System of Units Time measurement systems Decimal time de:Dezimalzeit