Question: The smallest division of time is measured in what units ?

text: Orders of magnitude (time)
Less than one second

An order of magnitude of time is (usually) a decimal prefix or decimal order-of-magnitude quantity together with a base unit of time, like a microsecond or a million years. In some cases, the order of magnitude may be implied (usually 1), like a second or year. In other cases, the quantity name implies the base unit, like century. In most cases, the base unit is seconds or years. Prefixes are not usually used with a base unit of years, so we say a million years, not a megayear. Clock time and calendar time have duodecimal or sexagesimal orders of magnitude rather than decimal, i.e. 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 largest realized amount of time, given known scientific data, is the age of the universe, about 13.8 billion years - the time since the Big Bang as measured in the cosmic microwave background rest frame. Those amounts of time together span 60 decimal orders of magnitude. Metric prefixes are defined spanning 10−24 to 1024, 48 decimal orders of magnitude which may be used in conjunction with the metric base unit of second. Metric units of time larger than the second are most commonly seen only in a few scientific contexts such as observational astronomy and materials science although this depends on author; for everyday usage and most other scientific contexts the common units of minutes (60 s), hours (3600 s or 3.6 ks), days (86 400 s), weeks, months, and years (of which there are a number of variations) are commonly used.
Unit ( s ) Multiple Symbol Definition Comparative examples & common units
10 1 Planck time t P Presumed to be the shortest theoretically measurable time interval ( but not necessarily the shortest increment of time - see quantum gravity ) 10 ys : One Planck time t P = ℏ G / c 5 { \displaystyle { \sqrt { \hbar G/c^ { 5 } } } } ≈ 5.39 × 10 s is the briefest physically meaningful span of time . It is the unit of time in the natural units system known as Planck units
10 1 yoctosecond ys Yoctosecond , ( yocto- + second ) , is one septillionth of a second 0.3 ys : mean lifetime of W and Z bosons 23 ys : Lifespan isotope 7 of hydrogen ( Hydrogen-7 ) 156 ys : mean lifetime of a Higgs Boson
10 1 zeptosecond zs Zeptosecond , ( zepto- + second ) , is one sextillionth of one second 2 zs : representative cycle time of gamma ray radiation released in the decay of a radioactive atomic nucleus ( here as 2 MeV per emitted photon ) 4 zs : cycle time of the zitterbewegung of an electron ( ω = 2 m e c 2 / ℏ { \displaystyle \omega =2m_ { e } c^ { 2 } /\hbar } )
10 1 attosecond as One quintillionth of one second 12 attoseconds : best timing control of laser pulses
10 1 femtosecond fs One quadrillionth of one second 1 fs : Cycle time for 300 nanometre light ; ultraviolet light ; light travels 0.3 micrometres ( µm ) . 140 fs : Electrons have localized onto individual bromine atoms 6Å apart after laser dissociation of Br 2 . 290 fs : Lifetime of a tauon
10 1 picosecond ps One trillionth of one second 1 ps : mean lifetime of a bottom quark ; light travels 0.3 millimeters ( mm ) 1 ps : typical lifetime of a transition state 4 ps : Time to execute one machine cycle by an IBM Silicon-Germanium transistor
10 1 nanosecond ns One billionth of one second 1 ns : Time to execute one machine cycle by a 1 GHz microprocessor 1 ns : Light travels 30 centimetres ( 12 in )
10 1 microsecond µs One millionth of one second 1 µs : Time to execute one machine cycle by an Intel 80186 microprocessor 2.2 µs : Lifetime of a muon 4-16 µs : Time to execute one machine cycle by a 1960s minicomputer
10 1 millisecond ms One thousandth of one second 1 ms : time for a neuron in human brain to fire one impulse and return to rest 4-8 ms : typical seek time for a computer hard disk
10 1 centisecond cs One hundredth of one second 1-2 cs ( =0.01-0.02 s ) : Human reflex response to visual stimuli 1.6667 cs period of a frame at a frame rate of 60 Hz . 2 cs : cycle time for European 50 Hz AC electricity
10 1 decisecond ds One tenth of a second 1-4 ds ( =0.1-0.4 s ) : Blink of an eye