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Clock time and calendar time have duodecimal or sexagesimal orders of magnitude rather than decimal, e.g., a year is 12 months, and a minute is 60 seconds. The smallest meaningful increment of time is the Planck time ―the time light takes to traverse the Planck distance , many decimal orders of magnitude smaller than a second.
The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, Δν Cs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s −1." [1] Historically, many units of time were defined by ...
A millisecond (from milli-and second; symbol: ms) is a unit of time in the International System of Units equal to one thousandth (0.001 or 10 −3 or 1 / 1000) of a second [1] [2] or 1000 microseconds.
The commission rejected the seconds-pendulum definition of the metre the following year because the second of time was an arbitrary period equal to 1/86,400 day, rather than a decimal fraction of a natural unit.
Unix time numbers are repeated in the second immediately following a positive leap second. The Unix time number 1 483 228 800 is thus ambiguous: it can refer either to start of the leap second (2016-12-31 23:59:60) or the end of it, one second later (2017-01-01 00:00:00). In the theoretical case when a negative leap second occurs, no ambiguity ...
It is also the standard single-unit time representation in many programming languages, most notably C, and part of UNIX/POSIX standards used by Linux, Mac OS X, etc.; to convert fractional days to fractional seconds, multiply the number by 86400. Fractional seconds are represented as milliseconds (ms), microseconds (μs) or nanoseconds (ns ...
Ephemeris Time was from 1952 to 1976 an official time scale standard of the International Astronomical Union; it was a dynamical time scale based on the orbital motion of the Earth around the Sun, from which the ephemeris second was derived as a defined fraction of the tropical year. This ephemeris second was the standard for the SI second from ...
Many computer systems measure time and date using Unix time, an international standard for digital timekeeping.Unix time is defined as the number of seconds elapsed since 00:00:00 UTC on 1 January 1970 (an arbitrarily chosen time based on the creation of the first Unix system), which has been dubbed the Unix epoch.