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. Weeks, months and years are significantly variable units whose length crucially depends on the choice of calendar and is often not regular even with a calendar, e.g. leap years versus regular years in the Gregorian calendar. This makes them problematic for use against a linear and regular time scale such as that defined by the SI since it is not clear as to which version of these units we are to be using. Because of this, in the table below we will not use weeks and months and the year we will use is the Julian year of astronomy, or 365.25 days of 86 400 s exactly, also called an annum and denoted with the symbol a, whose definition is based on the average length of a year of the Julian calendar which had one leap year every and always every 4 years against common years of 365 days each. This unit is used, following the convention of geological science, to form larger units of time by the application of SI prefixes to it at least up to giga-annum, or Ga, equal to 1 000 000 000 a (short scale: one billion years, long scale: one milliard years).
|Unit (s)||Multiple||Symbol||Definition||Comparative examples & common units|
|10−44||1 Planck time||tP||Presumed to be the shortest theoretically measurable time interval (but not necessarily the shortest increment of time - see quantum gravity)||: One Planck time tP = ?  is the briefest physically meaningful span of time. It is the unit of time in the natural units system known as Planck units.|
|10−24||1 yoctosecond||ys||Yoctosecond, (yocto- + second), is one septillionth of a second||0.3 ys: mean lifetime of W and Z bosons|
|10−21||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 ()
|10−18||1 attosecond||as||One quintillionth of one second||12 attoseconds: best timing control of laser pulses.|
|10−15||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 Br2.
290 fs: Lifetime of a tauon
|10−12||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
|10−9||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)
|10−6||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
|10−3||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
|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.
|ds||One tenth of a second||1–4 ds (=0.1–0.4 s): Blink of an eye|
In this table, large intervals of time surpassing one second are catalogued in order of the SI multiples of the second as well as their equivalent in common time units of minutes, hours, days, and Julian years.
|Unit (s)||Multiple||Symbol||Common units||Comparative examples & common units|
|101||1 decasecond||das||single seconds
(1 das = 10 s)
|6 das: one minute (min), the time it takes a second hand to cycle around a clock face|
(1 hs = 1 min 40 s = 100 s)
|2 hs (3 min 20 s): average length of the most popular YouTube videos as of January 2017|
5.55 hs (9 min 12 s): longest videos in above study
|103||1 kilosecond||ks||minutes, hours, days
(1 ks = 16 min 40 s = 1,000 s)
|1 ks: record confinement time for antimatter, specifically antihydrogen, in electrically neutral state as of 2011|
1.8 ks: time slot for the typical situation comedy on television with advertisements included
|106||1 megasecond||Ms||weeks to years
(1 Ms = 11 d 13 h 46 min 40 s = 1,000,000 s)
|1.641 6 Ms (19 d): length of a "month" of the Baha'i calendar|
2.36 Ms (27.32 d): length of the true month, the orbital period of the Moon
|109||1 gigasecond||Gs||decades, centuries, millennia
(1 Gs = over 31 years and 287 days = 1,000,000,000 s)
|1.5 Gs: UNIX time as of Jul 14 02:40:00 UTC 2017. UNIX time being the number of seconds since 1970-01-01T00:00:00Z ignoring leap seconds.|
2.5 Gs: (79 a): typical human life expectancy in the developed world
|1012||1 terasecond||Ts||millennia to geological epochs
(1 Ts = over 31,600 years = 1,000,000,000,000 s)
|3.1 Ts (100 ka): approximate length of a glacial period of the current Quaternary glaciation epoch|
31.6 Ts (1000 ka, 1 Ma): one mega-annum (Ma), or one million years
|1015||1 petasecond||Ps||geological eras, history of Earth and the Universe||2 Ps: approximate time since the Cretaceous-Paleogene extinction event, believed to be caused by the impact of a large asteroid into Chicxulub in modern-day Mexico. This extinction was one of the largest in Earth's history and marked the demise of most dinosaurs, with the only known exception being the ancestors of today's birds.|
7.9 Ps (250 Ma): approximate time since the Permian-Triassic extinction event, the actually largest known mass extinction in Earth history which wiped out 95% of all extant species and believed to have been caused by the consequences of massive long-term volcanic eruptions in the area of the Siberian Traps. Also, the approximate time to the supercontinent of Pangaea. Also, the length of one galactic year or cosmic year, the time required for the Sun to complete one orbit around the Milky Way Galaxy.
|1018||1 exasecond||Es||future cosmological time||All times of this length and beyond are currently theoretical as they surpass the elapsed lifetime of the known universe.|
1.08 Es (+34 Ga): time to the Big Rip according to some models, but this is not favored by existing data. This is one possible scenario for the ultimate fate of the Universe. Under this scenario, dark energy increases in strength and power in a feedback loop that eventually results in the tearing apart of all matter down to subatomic scale due to the rapidly increasing negative pressure thereupon
|1021||1 zettasecond||Zs||3 Zs (+100 000 Ga): The remaining time until the end of Stelliferous Era of the universe under the heat death scenario for the ultimate fate of the Universe which is the most commonly-accepted model in the current scientific community. This is marked by the cooling-off of the last low-mass dwarf star to a black dwarf. After this time has elapsed, the Degenerate Era begins.|
9.85 Zs (311 000 Ga): The entire lifetime of Brahma in Hindu mythology.
|1024and onward||1 yottasecond and beyond||Ys and on||600 Ys (9 × 1018 a): The radioactive half-life of bismuth-209 by alpha decay, one of the slowest-observed radioactive decay processes.|
1.310 019 × 1012 Ys (4.134 105 × 1028 years) - The time period equivalent to the value of 18.104.22.168.22.214.171.124.126.96.36.199.188.8.131.52.184.108.40.206.0.0.0.0 in the Mesoamerican Long Count, a date discovered on a stela at the Coba Maya site, believed by archaeologist Linda Schele to be the absolute value for the length of one cycle of the universe
1029 Ys (3.2×1045 years) - the largest possible value for the proton half-life, assuming that the Big Bang was inflationary and that the same process that made baryons predominate over antibaryons in the early Universe also makes protons decay