An order of magnitude of time is usually a decimal prefix or decimal orderofmagnitude 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. Therefore, it is said "a million years" instead of "a mega year". 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 largest realized amount of time, based on 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 to, 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 the author. For everyday use and most other scientific contexts, the common units of minutes, hours (3,600 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 depend on the choice of calendar and are 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 which version is being used.
Because of this, the table below does not include weeks, months, and years. Instead, the table uses the annum or astronomical Julian year (365.25 days of 86,400 seconds), denoted with the symbol a. Its definition is based on the average length of a year according to the Julian calendar, which has one leap year every four years. According to the geological science convention, this is used to form larger units of time by the application of SI prefixes to it; at least up to gigaannum or Ga, equal to 1,000,000,000 a (short scale: one billion years, long scale: one milliard years).
Unit  Symbol  Definition  Comparative examples & common units <   This is any multiple, within reason and which is commonly used, to express not only by comparative example the length of time but also, in other terms (common units) the interval of time. For example: 60 Seconds = 1 minute.>  

1 Planck time  t  Presumed to be the shortest theoretically measurable time interval (but not necessarily the shortest increment of time—see quantum gravity)  One Planck time t = \sqrt{\hbarG/c^{5}}  
1 yoctosecond  ys^{[2]}  Yoctosecond, (yocto + second), is one septillionth of a second  0.3 ys: mean lifetime of W and Z bosons 23 ys: Half life of isotope 7 of hydrogen (Hydrogen7) 156 ys: mean lifetime of a Higgs Boson  
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 ( \omega=2m_{e}c^{2/\hbar} 247 zs: an experimentallymeasured travel time of a photon across a hydrogen molecule, "for the average bond length of molecular hydrogen"^{[3]}  
1 attosecond  as  One quintillionth of one second  12 as: best timing control of laser pulses.^{[4]} 43 as: shortest laser pulse^{[5]}  
1 femtosecond  fs  One quadrillionth of one second  1 fs: Cycle time for 300nanometre 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.^{[6]} 290 fs: Lifetime of a tauon  
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 silicongermanium transistor 109 ps: Period of the photon corresponding to the hyperfine transition of the ground state of cesium133, and one 9,192,631,770th of one second by definition 114.6 ps: Time for the fastest overclocked processor to execute one machine cycle.^{[7]}  
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  
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  
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^{[8]} 4–8 ms: typical seek time for a computer hard disk  
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  
1 decisecond  ds  One tenth of a second  1–4 ds (=0.1–0.4 s): Blink of an eye^{[9]} 
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.
Multiple of a second  Unit  Symbol  Common units  Comparative examples & common units <   This is any multiple, within reason and which is commonly used, to express not only by comparative example the length of time but also, in other terms (common units) the interval of time. For example: 60 Seconds = 1 minute.>  

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 hectosecond  hs  minutes (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^{[10]} 5.55 hs (9 min 12 s): longest videos in above study 7.1 hs (11 m 50 s): time for a human walking at average speed of 1.4 m/s to walk 1 kilometre  
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^{[11]} 1.8 ks: time slot for the typical situation comedy on television with advertisements included 3.6 ks: one hour (h), time for the minute hand of a clock to cycle once around the face, approximately 1/24 of one mean solar day 7.2 ks (2 h): typical length of feature films 86.399 ks (23 h 59 min 59 s): one day with a removed leap second on UTC time scale. Such has not yet occurred. 86.4 ks (24 h): one day of Earth by standard. More exactly, the mean solar day is 86.400 002 ks due to tidal braking, and increasing at the rate of approximately 2 ms/century; to correct for this time standards like UTC use leap seconds with the interval described as "a day" on them being most often 86.4 ks exactly by definition but occasionally one second more or less so that every day contains a whole number of seconds while preserving alignment with astronomical time. The hour hand of an analogue clock will typically cycle twice around the dial in this period as most analogue clocks are 12hour, less common are analogue 24hour clocks in which it cycles around once. 86.401 ks (24 h 0 min 1 s): one day with an added leap second on UTC time scale. While this is strictly 24 hours and 1 second in conventional units, a digital clock of suitable capability level will most often display the leap second as 23:59:60 and not 24:00:00 before rolling over to 00:00:00 the next day, as though the last "minute" of the day were crammed with 61 seconds and not 60, and similarly the last "hour" 3601 s instead of 3600. 88.775 ks (24 h 39 min 35 s): one sol of Mars 604.8 ks (7 d): one week of the Gregorian calendar  
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 2.419 2 Ms (28 d): length of February, the shortest month of the Gregorian calendar 2.592 Ms (30 d): 30 days, a common interval used in legal agreements and contracts as a proxy for a month 2.678 4 Ms (31 d): length of the longest months of the Gregorian calendar 23 Ms (270 d): approximate length of typical human gestational period 31.557 6 Ms (365.25 d): length of the Julian year, also called the annum, symbol a. 31.558 15 Ms (365 d 6 h 9 min 10 s): length of the true year, the orbital period of the Earth 126.232 6 Ms (1461 d 0 h 34 min 40 s): the elected term of the President of the United States or one Olympiad  
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 19700101T00:00:00Z ignoring leap seconds. 2.5 Gs: (79 a): typical human life expectancy in the developed world 3.16 Gs: (100 a): one century 31.6 Gs: (1000 a, 1 ka): one millennium, also called a kiloannum (ka) 63.8 Gs: approximate time since the beginning of the Anno Domini era as of 2019 – 2,019 years, and traditionally the time since the birth of Jesus Christ 194.67 Gs: Approximate lifespan of time capsule Crypt of Civilization, 28 May 1940 – 28 May 8113 363 Gs: (11.5 ka): time since the beginning of the Holocene epoch 814 Gs: (25.8 ka): approximate time for the cycle of precession of the Earth's axis  
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 megaannum (Ma), or one million years 79 Ts (2.5 Ma): approximate time since earliest hominids of genus Australopithecus 130 Ts (4 Ma): the typical lifetime of a biological species on Earth 137 Ts (4.32 Ma): the length of the mythic unit of mahayuga, the Great Age, in Hindu mythology.  
1 petasecond  Ps  geological eras, history of Earth and the Universe  2 Ps: approximate time since the CretaceousPaleogene extinction event, believed to be caused by the impact of a large asteroid into Chicxulub in modernday 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 PermianTriassic 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 longterm 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. 16 Ps (510 Ma): approximate time since the Cambrian explosion, a massive evolutionary diversification of life which led to the appearance of most existing multicellular organisms and the replacement of the previous Ediacaran biota. 22 Ps (704 Ma): approximate halflife of the uranium isotope U. 31.6 Ps (1000 Ma, 1 Ga): one gigaannum (Ga), one billion years, the largest fixed time unit used in the standard geological time scale, approximately the order of magnitude of an eon, the largest division of geological time. +1 Ga: The estimated remaining habitable lifetime of Earth, according to some models. At this point in time the stellar evolution of the Sun will have increased its luminosity to the point that enough energy will be reaching the Earth to cause the evaporation of the oceans and their loss into space (due to the uv flux from the Sun at the top of the atmosphere dissociating the molecules), making it impossible for any life to continue. 136 Ps (4.32 Ga): The length of the legendary unit kalpa in Hindu mythology, or one day (but not including the following night) of the life of Brahma. 143 Ps (4.5 Ga): The age of the Earth by our best estimates. Also the approximate halflife of the uranium isotope U. 315 Ps (10 Ga): approximate lifetime of a mainsequence star similar to our Sun. 435 Ps (13.8 Ga): The approximate age of the Universe  
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 300 – 600 Es (10 000 – 20 000 Ga): The estimate lifetime of lowmass stars (red dwarfs)  
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 commonlyaccepted model in the current scientific community. This is marked by the coolingoff of the last lowmass 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.  
and onward  1 yottasecond and beyond  Ys and on  600 Ys : The radioactive halflife of bismuth209 by alpha decay, one of the slowestobserved radioactive decay processes. : The time period equivalent to the value of 13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.0.0.0.0 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^{[12]} ^{[13]} : the smallest possible value for proton halflife consistent with experiment^{[14]} : the largest possible value for the proton halflife, 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^{[15]}

6×10^{1} seconds  1 minute  m  

6×10^{1} minutes  1 hour  h (hr)  
2.4×10^{1} hours  1 day  d 