Metric time is the measure of time interval using the metric system, which defines the second as the base unit of time, and multiple and submultiple units formed with metric prefixes, such as kiloseconds and milliseconds. It does not define the time of day, as this is defined by various time scales, which may be based upon the metric definition of the second. Other units of time, the minute, hour, and day, are accepted for use with the modern metric system, but are not part of it.
When the metric system was introduced in France in 1795, it included units for length, area, dry volume, liquid capacity, weight or mass, and even currency, but not for time. Decimal time of day had been introduced in France two years earlier, but was set aside at the same time the metric system was inaugurated, and did not follow the metric pattern of a base unit and prefixed units. James Clerk Maxwell and Elihu Thomson (through the British Association for the Advancement of Science - BAAS) introduced the Centimetre gram second system of units (cgs) in 1874, in order to derive electric and magnetic metric units, following the recommendation of Carl Friedrich Gauss in 1832.
The ephemeris second (defined as 1/86400 of a mean solar day) was made one of the original base units of the modern metric system, or International System of Units (SI), at the 10th General Conference on Weights and Measures (CGPM) in 1954. 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.
Numerous proposals have been made for alternative base units of metric time. On March 28, 1794, the president of the commission which developed the metric system, Joseph Louis Lagrange, proposed in a report to the commission the names déci-jour and centi-jour (deciday and centiday in English). Base units equivalent to decimal divisions of the day, such as 1/10, 1/100, 1/1000, or 1/100,000 day, or other divisions of the day, such as 1/20 or 1/40 day, have also been proposed, with names such as tick, meck, chi, chron, moment, etc., and multiple and submultiple units formed with metric prefixes. Such alternative units have not gained any notable acceptance, however, mostly from sheer lack of acquaintance and familiarity.
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