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.
A modified second = 1/100 000 of a day = 0.864 s could be a viable alternative. Any redefinition of the second, however, creates conflicts with anything based on its precise current definition. Another unit for time, more familiar than some other suggestions, could be 14.4 minutes, i.e. a shorter quarter of an hour, or a centiday, as proposed by Lagrange. The centiday was used in China (called ke in Chinese) for thousands of years, until the Jesuits had it redefined from 1/100th of a day to 1/96th of a day (i.e., 15 minutes) in the 17th century.
In 1897, the 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.
Metric time is sometimes used to mean decimal time. Metric time properly refers to measurement of time interval, while decimal time refers to the time of day. Standard time of day is defined by various time scales, such as UTC, which are now usually based upon the metric base unit of time, the second. Some proposals for alternative units of metric time are accompanied by decimal time scales for telling the time of day based upon these alternative units. Other proposals called "metric time" refer only to decimal time, and therefore are not truly metric.
French decimal time is sometimes called "metric time" because it was introduced around the same time as the metric system and both were decimal, but it was not part of the decree creating the original metric system and its units were named for the hour, minute and second, instead of using metric prefixes. Other decimal time standards, such as Swatch Internet Time, are not considered metric time.
In computing, at least internally, metric time gained widespread use for ease of computation. Unix time gives date and time almost as number of seconds since January 1 1970, and Microsoft's FILETIME almost as multiples of 100ns since January 1 1601 . VAX/VMS uses the number of 100ns since November 17 1858 and RISC OS the number of centiseconds since January 1, 1900. (Each of these is not strictly linear, as they have discontinuities at leap seconds.)
The main problem of metric time lies in the units. The International System of Units has only developed prefixes regarding 10 units exponentially in both the multiple and submultiple directions. The first three multiples would be viable for use within a metric time system; they are 101 (decasecond = 10 seconds), 102 (hectosecond = 100 seconds; 1.666 minutes) and 103 (kilosecond = 1 000 seconds; 16.666 minutes) respectively. However, the fourth value in the SI Units is 106 (megasecond = 1 000 000 seconds; 16 666.666 minutes; 277.777 hours; 11.574 days). Followed by 109 (gigasecond = 1 000 000 000 seconds; 16 666 666.666 minutes; 277 777.777 hours; 11 574.074 days; 31.689 years); a relatively unviable unit to measure human time/life in. To make this system of prefixes work for metric time, standard units and prefixes would have to be developed for the 4th and 5th exponents to make metric time viable to human life.