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{{short description|Measurement unit for time}}
{{more citations needed|date=March 2016}}
[[File:Units of Time in tabular form.png|thumb|Table showing quantitative relationships between common units of time]]

A '''unit of time''' is any particular [[time]] interval, used as a standard way of measuring or expressing duration. The [[SI base unit|base unit]] of time in the [[International System of Units]] (SI), and by extension most of the [[Western world]], is the [[second]], defined as about 9 billion oscillations of the [[caesium]] atom. The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, {{math|Δ''ν''<sub>Cs</sub>}}, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be {{val|9192631770}} when expressed in the unit Hz, which is equal to s<sup>−1</sup>."<ref>{{cite web |title=The International System of Units – 9th edition – Complete text in English and French (2019) |url=https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf/fcf090b2-04e6-88cc-1149-c3e029ad8232?version=1.18&t=1645193776058&download=true |website=BIPM |access-date=22 July 2022}}</ref>

Historically, many units of time were defined by the movements of [[astronomical object]]s.
* [[Sun]]-based: the [[year]] is based on the [[Earth's orbit|Earth's orbital period]] around the sun. Historical year-based units include the [[Olympiad]] (four years), the [[lustrum]] (five years), the [[indiction]] (15 years), the [[decade]], the [[century]], and the [[millennium]].
* [[Moon]]-based: the [[month]] is based on the [[Orbit of the Moon|Moon's orbital period]] around the Earth.
* [[Earth]]-based: the [[day]] is based on the time it takes for the Earth to rotate on its own axis, relative to the Sun. Units originally derived from this base include the [[week]] (seven days), and the [[fortnight]] (14 days). Subdivisions of the day include the [[hour]] (1/24 of a day), which is further subdivided into [[minute]]s and seconds. The second is the international standard unit (SI unit) for science. 
* [[Celestial sphere]]-based: as in [[sidereal time]], where the apparent movement of the stars and [[constellations]] across the sky is used to calculate the length of a year.

These units do not have a consistent relationship with each other and require [[Intercalation (timekeeping)|intercalation]]. For example, the year cannot be divided into twelve 28-day months since 12 times 28 is 336, well short of 365. The [[lunar month]] (as defined by the moon's rotation) is not 28 days but 28.3 days. The year, defined in the [[Gregorian calendar]] as {{val|365.2425}} days has to be adjusted with [[leap days]] and [[leap seconds]]. Consequently, these units are now all defined for scientific purposes as multiples of seconds.

Units of time based on [[Time (Orders of magnitude)|orders of magnitude]] of the second follow the system of [[metric_prefix|metric prefixes]].

== Historical ==
{{main|History of calendars}}
The natural units for timekeeping used by most historical societies are the [[day]], the [[solar year]] and the [[lunation]]. Such calendars include the [[Sumerian calendar|Sumerian]], [[Egyptian calendar|Egyptian]], [[Chinese calendar|Chinese]], [[Babylonian calendar|Babylonian]], [[Attic calendar|ancient Athenian]], [[Buddhist calendar|Buddhist]], [[Hindu calendar|Hindu]], [[Islamic calendar|Islamic]], [[Icelandic calendar|Icelandic]], [[Mayan calendar|Mayan]], and [[French Republican calendar|French Republican]] calendars.

The modern calendar has its origins in the [[Roman calendar]], which evolved into the [[Julian calendar]], and then the [[Gregorian calendar]].

{{wide image|Logarithmic time scale - milliseconds to years.svg|1200px|Horizontal [[logarithmic scale]] marked with units of time in the [[Gregorian calendar]]}}

== Scientific ==
* The [[Planck time]] is the time that light takes to travel one [[Planck length]].
* The [[Jiffy (time)|Jiffy]] is the amount of time light takes to travel one [[femtometre]] (about the diameter of a [[nucleon]]).
* The [[atomic_units|atomic time]] relates to the orbital period of a ground state [[electron]] around a [[hydrogen atom]] and is about 24.2 attoseconds.
* The [[svedberg]] is a time unit used for [[sedimentation]] rates (usually of proteins). It is defined as 10<sup>−13</sup> seconds (100&nbsp;fs).
* The [[TU (time unit)|TU]] (for ''time unit'') is a unit of time defined as 1024&nbsp;μs for use in engineering.
* The [[galactic year]], based on the rotation of the galaxy and usually measured in million years.<ref name="StarChild">http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000</ref>
* The [[geological time scale]] relates [[stratigraphy]] to time. The [[deep time]] of Earth's past is divided into units according to events that took place in each period. For example, the boundary between the [[Cretaceous]] period and the [[Paleogene]] period is defined by the [[Cretaceous–Paleogene extinction event]]. The largest unit is the supereon, composed of eons. Eons are divided into [[Era (geology)|eras]], which are in turn divided into [[Period (geology)|periods]], [[Epoch (geology)|epochs]] and [[Age (geology)|ages]]. It is not a true mathematical unit, as all ages, epochs, periods, eras, or eons don't have the same length; instead, their length is determined by the geological and historical events that define them individually.

Note: The [[light-year]] is not a unit of time, but a unit of length of about 9.5 petametres ({{val|9454254955488|u=km}}).

Note: The [[parsec]] is not a unit of time, but a unit of length of about 30.9 trillion kilometres, despite [[Millennium Falcon#Kessel Run|movie references]] otherwise.

== List ==
<!-- Please do not needlessly inflate this list with contrived, obscure, and/or unsourced material.  -->{| class="wikitable mw-collapsible"
|+ {{nowrap|Units of time}}
!Name
!Length
!Notes
|-
| [[Planck time]] || ~{{val|5.39|e=−44|ul=s}} || The amount of time light takes to travel one [[Planck length]].
|-
| [[Orders of magnitude (time)|quectosecond]] || {{val|e=-30|ul=s}} || One nonillionth of a second.
|-
| [[Orders of magnitude (time)|rontosecond]] || {{val|e=-27|ul=s}} || One octillionth of a second.
|-
| [[yoctosecond]] || {{val|e=-24|ul=s}} || One septillionth of a second.
|-
| [[Jiffy (time)|jiffy]] (physics) || {{val|3e−24|ul=s}} || The amount of time light takes to travel one [[Fermi (length)|fermi]] (about the size of a nucleon) in a vacuum.
|-
| [[zeptosecond]] || {{val|e=−21|ul=s}} || One sextillionth of a second. Time measurement scale of the NIST and JILA strontium atomic clock. Smallest fragment of time currently measurable is 247 zeptoseconds.<ref>{{Cite news|url=https://www.livescience.com/zeptosecond-shortest-time-unit-measured.html/|title=Meet the zeptosecond, the shortest unit of time ever measured|access-date=2020-10-17|language=en-EN}}</ref>
|-
| [[attosecond]] || {{val|e=−18|ul=s}} || One quintillionth of a second.
|-
| [[atomic_unit|atomic time]] || ~{{val|2.42|e=−17|ul=s}} || Derived from atomic theory of hydrogen.
|-
| [[femtosecond]] || {{val|e=−15|ul=s}} || One quadrillionth of a second.
|-
| [[svedberg]] || {{val|e=−13|ul=s}} || 100 femtoseconds, time unit used for sedimentation rates (usually of proteins).
|-
| [[picosecond]] || {{val|e=−12|ul=s}} || One trillionth of a second.
|-
| [[nanosecond]] || {{val|e=−9|ul=s}} || One billionth of a second. Time for molecules to fluoresce.
|-
| [[Shake (unit)|shake]] || {{val|e=−8|ul=s}} || 10 nanoseconds, also a casual term for a short period of time.
|-
| [[microsecond]] || {{val|e=−6|ul=s}}|| One millionth of a second. Symbol is μs
|-
| [[millisecond]] || {{val|e=-3|ul=s}} || One thousandth of a second. Shortest time unit used on stopwatches.
|-
| [[Orders of magnitude (time)|centisecond]] || {{val|e=-2|ul=s}} || One hundredth of a second.
|-
| [[Jiffy (time)|jiffy]] (electronics) ||~{{val|2|e=-2|ul=s}} 
| Used to measure the time between alternating power cycles.
|-
| [[Orders of magnitude (time)|decisecond]] || {{val|e=-1|ul=s}} || One tenth of a second.
|-
|- style="background-color:rgba(255,255,0,0.25);"
| [[second]] || 1 s || [[SI base unit]] for time.
|-
| [[Orders of magnitude (time)|decasecond]] || {{val|10|ul=s}} ||Ten seconds (one sixth of a minute)
|-
| [[minute]] || {{val|60|ul=s}} ||
|-
| [[Orders of magnitude (time)|hectosecond]] || {{val|100|ul=s}} ||
|-
| [[Swatch Internet Time|milliday]] || {{val|1|/|1000|ul=d}} ({{val|0.001|ul=d}})
| 1.44 minutes, or 86.4 seconds. Also marketed as a "[[Swatch Internet Time|.beat]]" by the Swatch corporation.
|-
| [[moment (unit)|moment]] || {{val|1|/|40|ul=solar hour}} ({{val|90|ul=s}} on average) || Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season.<ref>{{cite book |last=Milham |first=Willis I. |title=Time and Timekeepers |year=1945 |publisher=MacMillan |location=New York |pages=190 |isbn=0-7808-0008-7}}</ref> Also colloquially refers to a brief period of time.
|-
|centiday
|{{val|0.01|ul=d}} (1 % of a day)
|14.4 minutes, or 864 seconds. One-hundredth of a day is 1 cd (centiday), also called "''kè''" in traditional [[Traditional Chinese timekeeping#One-hundredth of a day: kè|Chinese timekeeping]]. The unit was also proposed by Lagrange and endorsed by Rey-Pailhade<ref>{{Cite thesis |last=Gamez |first=Christophe |title=La décimalisation du temps au prisme du Bureau des longitudes (1875-1901). Entre patriotisme, rationalité et politique |date=2020-06-17 |degree=other |publisher=Université de Lorraine |url=https://hal.univ-lorraine.fr/hal-04376271v1 |language=fr}}</ref> in the 19th century, named "''[[Metric time#History|centijours]]''" (from French ''centi-'' 'hundred' and ''jour'' 'day').
|-
| [[kilosecond]] || {{val|e=3|ul=s}} || About 17 minutes.
|-
| [[hour]] || {{val|60|ul=min}} ||
|-
|deciday
|{{val|0.1|ul=d}} (10 % of a day)
|2.4 hours, or 144 minutes. One-tenth of a day is 1 dd (deciday), also called "''gēng''" in traditional [[Traditional Chinese timekeeping#One-tenth of a day: gēng|Chinese timekeeping]].
|-
| [[day]] || {{val|24|ul=h}} || Longest unit used on stopwatches and countdowns. The SI day is exactly 86 400 seconds.
|-
| [[week]] || {{val|7|ul=d}} || Historically sometimes also called "sennight".
|-
|decaday
|{{val|10|ul=d}} ({{val|1|u=Dd}})
|10 days. A period of time analogous to the concept of "week", used by [[Week#"Weeks" in other calendars|different societies]] around the world: the ancient [[Egyptian calendar]], the ancient [[Chinese calendar#Week|Chinese calendar]], and also the [[French Republican calendar]] (in which it was called a ''décade'').
|-
| [[megasecond]] || {{val|e=6|ul=s}} || About 11.6 days.
|-
| [[fortnight]] || {{val|2|ul=weeks}} || 14 days
|-
| [[lunar month]] || {{val|27|ul=d}}{{nbsp}}{{val|4|ul=h}}{{nbsp}}{{val|48|ul=min}}{{snd}}{{val|29|ul=d}}{{nbsp}}{{val|12|ul=h}} || Various definitions of ''lunar month'' exist; sometimes also called a "lunation".
|-
| [[month]] || {{val|28|{{ndash}}|31|ul=d}} || Occasionally calculated as 30 days.
|-
|[[Isolation (health care)|quarantine]]
|40 [[Day|d]] (approximately 5.71 [[week]]s)
|To retain in obligatory isolation or separation, as a sanitary measure to prevent the spread of contagious disease. Historically it meant to be isolated for [[40 (number)|40]] days. From Middle English quarantine, from [[Italian language|Italian]] ''quarantina'' ("forty days"), the period Venetians customarily kept ships from plague-ridden countries waiting off port, from ''quaranta'' ("forty"), from [[Latin]] ''quadrāgintā''.
|-
|hectoday
|{{val|100|ul=d}} ({{val|1|u=hd}})
|100 days, roughly equivalent to 1/4 of a year (91.25 days). In Chinese tradition "''bǎi rì''" ([[wiktionary:百日|百日]]) is the hundredth day after one's birth, also called Baby's 100 Days Celebration.
|-
| [[semester]] || {{val|18|ul=weeks}} || A division of the academic year.<ref>{{cite web|url=http://www.merriam-webster.com/dictionary/semester|title=Semester|publisher=Webster's Dictionary|access-date=3 December 2014}}</ref> Literally "six months", also used in this sense.
|-
| [[lunar year]] || {{val|354.37|ul=d}} ||
|-
| [[year]] || {{val|12|u=[[Month|mo]]}} || {{val|365|or|366|ul=days}}
|-
| [[common year]] || {{val|365|ul=d}} || 52 weeks and 1 day.
|-
| [[tropical year]] || {{val|365|ul=d}}{{nbsp}}{{val|5|ul=h}}{{nbsp}}{{val|48|ul=min}}{{nbsp}}{{val|45.216|ul=s}}<ref>{{cite book
|title=Time: from Earth rotation to atomic physics
|first1=Dennis D.
|last1=McCarthy
|first2=P. Kenneth
|last2=Seidelmann
|publisher=Wiley-VCH
|year=2009
|isbn=978-3-527-40780-4
|page=18
|url=https://books.google.com/books?id=7NdrK4e77CIC}}, [https://books.google.com/books?id=7NdrK4e77CIC&pg=PA18 Extract of page 18]
</ref> ||Average.
|-
| [[Gregorian calendar|Gregorian year]] || {{val|365|ul=d}}{{nbsp}}{{val|5|ul=h}}{{nbsp}}{{val|49|ul=min}}{{nbsp}}{{val|12|ul=s}} ||Average.
|-
| [[sidereal year]] || {{val|365|ul=d}}{{nbsp}}{{val|6|ul=h}}{{nbsp}}{{val|9|ul=min}}{{nbsp}}{{val|9.7635456|ul=s}}
|-
| [[leap year]] || {{val|366|ul=d}} || {{val|52|ul=weeks}} and {{val|2|ul=days}}
|-
|{{anchor|quadrennium}} [[olympiad]] || {{val|4|ul=yr}} || A quadrennium (plural: quadrennia or quadrenniums) is also a period of four years, most commonly used in reference to the four-year period between each [[Olympic Games]].<ref>{{cite web|title=Merriam-Webster Dictionary|url=http://www.merriam-webster.com/dictionary/quadrennium|publisher=Merriam-Webster Incorporated|access-date=29 November 2016}}</ref> It is also used in reference to the four-year interval between [[leap year]]s, for example when wishing friends and family a "happy quadrennium" on [[February 29]].
|-
| [[lustrum]] || {{val|5|ul=yr}}  || In early Roman times, the interval between censuses.
|-
| [[decade]] || {{val|10|ul=yr}}  ||
|-
| [[indiction]] || {{val|15|ul=yr}}  || Interval for taxation assessments (Roman Empire). <!-- Please refer to the talk page before adding "score" as a unit of twenty years.  -->
|-
| [[gigasecond]] || {{val|e=9|ul=s}} || About 31.7 years.
|-
| [[Jubilee (biblical)|jubilee]] || {{val|50|ul=yr}}
|
|-
| [[century]] || {{val|100|ul=yr}}  ||
|-
| [[millennium]] || {{val|1,000|ul=yr}} || Also called "kiloannum".
|-
| [[Astrological age|Age]] || 2 148 and 2/3 of a year|| A [[Superstition|superstitious]] unit of time used in [[astrology]], each of them representing a star sign.
|-
| [[Terasecond and longer|terasecond]] || {{val|e=12|ul=s}} || About 31,709 years.
|-
| [[Wiktionary:mega-annum|megaannum]] || {{val|e=6|ul=yr}} || Also called "[[megayear]]". 1000 millennia (plural of millennium), or 1 million years (in [[geology]], abbreviated as ''Ma'').
|-
| [[Terasecond and longer|petasecond]] || {{val|e=15|ul=s}} || About {{val|31,709,791}} years.
|-
| [[galactic year]] || {{val|2.3|e=8|ul=yr}} ||The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy (approx {{val|230,000,000}} years<ref name="StarChild" />).
|- style="background-color:rgba(220,220,220,0.50);"
| [[cosmological decade]] || logarithmic (varies) ||10 times the length of the previous cosmological decade, with CD 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition.
|-
| [[aeon|eon]] || {{val|e=9|ul=yr}} || Also refers to an indefinite period of time, otherwise is {{val|1,000,000,000}} years.
|-
| [[Kalpa (time)|kalpa]] || {{val|4.32|e=9|ul=yr}} || Used in [[Hindu mythology]]. About {{val|4,320,000,000}} years.
|-
| [[Terasecond and longer|exasecond]] || {{val|e=18|ul=s}} || About {{val|31,709,791,983}} years. Approximately 2.3 times the current [[age of the universe]].
|-
| [[Terasecond and longer|zettasecond]] || {{val|e=21|ul=s}} || about {{val|31,709,791,983,764}} years.
|-
| [[Terasecond and longer|yottasecond]] || {{val|e=24|ul=s}} || About {{val|31,709,791,983,764,586}} years.
|-
| [[Terasecond and longer|ronnasecond]] || {{val|e=27|ul=s}} || About {{val|31,709,791,983,764,586,504}} years.
|-
| [[Terasecond and longer|quettasecond]] || {{val|e=30|ul=s}} || About {{val|31,709,791,983,764,586,504,312.1}} years.
|-
|}
<!-- Please do not needlessly inflate this list with contrived, obscure, and/or unsourced material.  -->

== Interrelation ==
[[File:Time units.svg|thumb|Flowchart illustrating selected units of time. The graphic also shows the three celestial objects that are related to the units of time.]]
All of the formal units of time are scaled multiples of each other. The most common units are the second, defined in terms of an atomic process; the day, an integral multiple of seconds; and the [[year]], usually 365 days. The other units used are multiples or divisions of these 3.

== See also ==
* [[Unit of frequency]]
* [[Orders of magnitude (time)]]

== References ==
{{reflist}}
{{Time topics}}
{{Time measurement and standards}}

[[Category:Units of time| ]]