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[[File:Units of Time in tabular form.png|thumb|Table showing quantitative relationships between common units of time]]

A '''unit of time''' or '''midst unit''' 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.

Note: The [[light-year]] is not a unit of time, but a unit of length of about 9.5 petametres (9 454 254 955 488 kilometers).

==List==
{| class="wikitable mw-collapsible"
|+ {{nowrap|Units of time}} 
|-
!scope="col"| Unit
!scope="col"| Length, Duration and Size
!scope="col"| Notes
|-
| [[Planck time]] || {{val|5.39|e=−44|ul=s}} || The amount of time light takes to travel one [[Planck length]].
|-
| [[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 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.
|-
| [[femtosecond]] || {{val|e=−15|ul=s}} || One quadrillionth of a second. Pulse time on fastest lasers.
|-
| [[svedberg]]|| {{val|e=−13|ul=s}}|| 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
|-
| centimillisecond || {{val|e=−5|ul=s}}|| One hundred thousandth of a second or one hundredth of a millisecond.
|-
| decimillisecond || {{val|e=−4|ul=s}}|| One ten-thousandth of a second or one tenth of a millisecond.
|-
| [[millisecond]] || {{val|e=-3|ul=s}} || One thousandth of a second. Shortest time unit used on stopwatches.
|-
| [[centisecond]] || {{val|e=-2|ul=s}} || One hundredth of a second.
|-
| [[Jiffy (time)|jiffy]] (electronics) || {{val|1|/|60|ul=s}} or {{val|1|/|50|ul=s}} || Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
|-
| [[decisecond]] || {{val|e=-1|ul=s}} || One tenth of a second.
|-
| quadrisecond / semisemisecond || {{val|0.25|ul=s}} || One quarter of a second.
|-
| semisecond / half second || {{val|0.5|ul=s}} || One half of a second.
|-
| [[second]] || {{val|1|ul=s}} || [[SI]] Base unit.
|-
| [[decasecond]] || {{val|10|ul=s}} ||
|-
|[[half a minute]] || {{val|30|ul=s}} || It’s known by math.
|-
| [[minute]] || {{val|60|ul=s}} ||
|-
| [[Swatch_Internet_Time|milliday]] || {{val|1|/|1000|ul=d}} || Also marketed as a ".beat" by the Swatch corporation; Milliday is a more accurate name.
|-
| [[moment (time)|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>
|-
| [[hectosecond]] || {{val|100|ul=s}} || 1 minute and 40 seconds
|-
| ke || {{val|864|ul=s}} || 14 minutes and 24 seconds
|-
| [[kilosecond]] || {{val|1000|ul=s}} || 16 minutes and 40 seconds
|-
|[[half an hour]] || {{val|30|ul=min}} ||It’s known by math.
|
|
|-
| [[hour]] || {{val|60|ul=min}} ||
|-
| decakilosecond || {{val|e=4|ul=s}} || Ten thousand seconds or ten kiloseconds.
|-
|[[Half an day]] || {{val|12|ul=h}}
|Its known by math.
|-
| [[day]] || {{val|24|ul=h}} || Longest unit used on stopwatches and countdowns.
|-
| hectokilosecond || {{val|e=5|ul=s}} || One hundred thousand seconds or one hundred kiloseconds.
|-
| [[week]] || {{val|7|ul=d}} || Also called "sennight".
|-
| [[megasecond]] || {{val|e=6|ul=s}} || 277.777778333333 hours or about 1 week and 4.6 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.
|-
| [[month]] || 28-31 days || Occasionally calculated as 30 days.
|-
| [[Fiscal year|quarter and season]] || {{val|3|u=[[Month|mo]]}} ||
|-
| [[Wiktionary:quadrimester|quadrimester ]]|| {{val|4|u=[[Month|mo]]}} ||
|-
| [[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.
|-
| half year || {{val|6|u=[[Month|mo]]}} ||
|-
| [[lunar year]] || {{val|354.37|u=[[days]]}} ||
|-
| [[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=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}}
|-
| [[wikt:biennial|biennium]] || {{val|2|ul=yr}}  ||
|-
| [[triennium]]|| {{val|3|ul=yr}}  ||
|-
| quadrennium || {{val|4|ul=yr}}  ||
|-
| [[olympiad]]|| {{val|4|ul=yr}}  ||
|-
| [[lustrum]]|| {{val|5|ul=yr}}  ||
|-
| [[decade]] || {{val|10|ul=yr}}  ||
|-
| [[indiction]]|| {{val|15|ul=yr}}  ||
|-
| [[gigasecond]] || {{val|e=9|ul=s}} || 16,666,666.6667 minutes or about 31.7 years.
|-
| [[Jubilee (biblical)|jubilee]]|| {{val|50|ul=yr}}
|-
| [[century]] || {{val|100|ul=yr}}  ||
|-
| [[millennium]] || {{val|1000|ul=yr}} || Also called "kiloannum".
|-
| [[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]]." 1,000 millennia (plural of millennium), or 1 million years.
|-
| [[Terasecond and longer|petasecond]] || {{val|e=15|ul=s}} || About 31,709,791 years.
|-
| [[galactic year]]|| {{val|2.3e8|ul=yr}}||The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time. Around 230,000,000 years.
|-
| [[cosmological decade]]|| varies ||<small>10 times the length of the previous<br/>cosmological decade, with CÐ 1 beginning<br/>either 10 seconds or 10 years after the<br/>Big Bang, depending on the definition.</small>
|-
| [[aeon|gigaannum]]|| {{val|e=9|ul=yr}} || Also refers to an indefinite period of time, otherwise is 1,000,000,000 years.
|-
| [[Terasecond and longer|exasecond]] || {{val|e=18|ul=s}} || About 31,709,791,983 years.
|-
| [[Terasecond and longer|zettasecond]] || {{val|e=21|ul=s}} || About 31,709,791,983,764 years.
|-
| [[Terasecond and longer|yottasecond]] || {{val|e=24|ul=s}} || About 31,709,791,983,764,584 years.
|}
| -
| [[The Age Of The Universe]] || {{val|13700000000|ul=yr}} || Alternatively, it’s roughly 4.35 yottaseconds
|}

==References==
{{reflist}}

[[Category:Units of time]]