Metric time is a way of measuring how long a period of time is using the metric system. The metric system uses the second as a main unit of time. There are periods of time that are based on the second but shorter. They are named with metric prefixes. Some of these are kiloseconds and milliseconds. Other units of time, like the minute, hour, and day, are accepted for use with the modern metric system, but are not part of it.
The metric system was first used in France in 1795. It had units for length, area, dry volume, liquid capacity, weight or mass, and even amounts of money, but not for time. Decimal time of day had been first used in France two years earlier, but was set aside at the same time the metric system was started, 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) came up with the Centimetre gram second system of units (cgs) in 1874. The ephemeris second (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 made even more exact. It was made into the time of 9,192,631,770 periods of the radiation from the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
Other units [change]
People have had ideas for other base units of metric time (other than the second). On March 28, 1794, Joseph Louis Lagrange, the president of the commission which developed the metric system, wrote a report to the commission. His report suggested the names déci-jour and centi-jour (deciday and centiday in English). Base units that are 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 suggested, with names such as tick, meck, chi, chron, moment, etc. These other units have not caught on with people, however.
In computing [change]
In computing, metric time became widely used because it is easy to use with computers. Unix time gives date and time as the number of seconds since January 1, 1970, and Microsoft's FILETIME 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 exactly linear, as there are some interruptions with leap seconds.)
The main problem with metric time is in the units. The International System of Units has only come up with prefixes for 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 nonviable unit with which to measure human time/life. To make this system of prefixes work for metric time, standard units and prefixes would have to be made for the 4th and 5th exponents to make metric time work for talking about time of human life. These would be:
104 seconds (10 000 seconds; 166.6 minutes; 2.777 hours)
105 seconds (100 000 seconds; 1 666.6 minutes; 27.77 hours; 1.157 days)