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.
Although part of the decimal metric system, the second derives its name from the sexagesimal system, which originated with the Sumerians and Babylonians, and divides a base unit into sixty minutes, minutes into sixty seconds, seconds into sixty thirds, etc. The word "minute" comes from the Latin pars minuta prima, meaning first small part, and "second" from pars minuta secunda or second small part. Coincidentally, angular measure also uses these sexagesimal units; in that field, it is the degree that is subdivided into minutes and seconds, while in time, it is the hour.
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.
A decimal second = 1/100 000 of a day = 0.864 s is an obvious 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.
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.
Swatch Internet Time introduced a time unit called .beat, equal to the French decimal minute or 1/1000 day. However, there are no other units with metric prefixes.
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 usually measured by the 24-hour clock or its closely related derivative, the 12-hour clock. These measurement systems are now based on 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. Other proposals called "metric time" refer only to decimal time and are therefore 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. The April 7, 1795 decree creating the original metric units and prefixes actually suspended decimal time, which had named its units the (decimal) hour, minute and second instead of using metric prefixes.
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 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.
None of these systems is strictly linear, as they each have discontinuities at leap seconds.
Fractions of a second
While larger metric prefixes of a second are rarely used, subdivision prefixes are well established within science and technology. Milliseconds, microseconds and smaller units (but not centiseconds) are well established.
The main problem lies in the size of time units created using metric prefixes. The International System of Units has developed 10 prefixes to express units as exponential multiples or sub-multiples. The first three multiples would be viable for use within a metric time system; they are:
|Multiple||Name of Unit||Seconds||Minutes|
However, the next multiples are:
|Multiple||Name of Unit||Seconds||Minutes||Hours||Days||Years|
|106||megasecond||1 000 000||16 666.666||277.777||11.574||0.032|
|109||gigasecond||1 000 000 000||16 666 666.666||277 777.777||11 574.074||31.688|
Such lengths of time are relatively impractical for measuring human activities.
For decimal multiples of the metric time unit to be practical, standard units and prefixes would need to be developed or revived for the 4th and 5th exponents so that it becomes viable for measuring human life. Historically, the metric system included a myria- prefix to represent 104, but its use was abrogated when the SI was introduced in 1960. 105 was represented using various ad hoc solutions; 100 000 meters composed a grade, 100 000 pascals composed a bar, and 100 000 dynes composed a newton; most of these conversions stem from compatible units that were used in the SI (and its predecessor, MKS) and the CGS system of units. Meteorologists currently use a unit of 100 000 seconds (a quantity on the order of a day) to quantify atmospheric vorticity at the synoptic scale; the unit, however, does not have a name.
|Multiple||Name of Unit||Seconds||Minutes||Hours||Days|
|105||metric day||100 000||1 666.6||27.77||1.157|
In popular culture
Metric time appears in science fiction writing on occasion. In the novel A Deepness in the Sky by Vernor Vinge, the spacefaring Qeng Ho culture is depicted as using metric units such as the kilosecond and megasecond for all time interval measurements.
The novels Accelerando and Glasshouse by Charles Stross have several uses of metric time, including kiloseconds, megaseconds, and megayears. In the second case, humans are living in habitats circling brown dwarfs, with no lingering connection to the Solar System that gave birth to the earlier time scale. In Glasshouse characters use kiloseconds, "megs" etc. to refer to different periods of time, any earlier use of megayears has been dropped.
In television, the Simpsons episode "They Saved Lisa's Brain" has Principal Skinner saying that, thanks to the city being under Mensa's control, the city's trains are not only running on time, but they are running on metric time, while looking at an analog clock with numbers 1–10 (which indicates decimal time).
In 2014, WestJet Airlines created an April Fool's Day joke stating they were switching to metric time, including how to calculate.
- Procès-verbaux du Comité d'instruction publique de la Convention nationale by James Guillaume
- Histoire d'heure - Fractionnement du temps
- AJB, Volume 9, 1907
- Report of the Sixth International Geographical Congress: Held in London, 1895
- Einstein's Clocks, Poincare's Maps: empires of time By Peter Louis Galison
- FILETIME documentation on MSDN
- "They Saved Lisa's Brain". The Simpsons. Season 10. Episode 22. 1999-05-09. Fox.
- Metric unit of time (second) Official text of SI brochure from International Bureau of Weights and Measures
- Metric Time? University of Illinois Physics Department
-  Westjet Switching to Metric Time page