Unit of time


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 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 definition, from the National Institute of Standards and Technology is: "The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the cesium frequency ΔνCs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1."[1]

Historically, many units of time were defined by the movements of astronomical objects.

  • Sun-based: the year was the time for the Earth to revolve 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 was based on the Moon's orbital period around the Earth.
  • Earth-based: the time it took for the Earth to rotate on its own axis, as observed on a sundial[citation needed]. 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 was further subdivided into minutes and finally seconds. The second became the international standard unit (SI units) 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. 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 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 orders of magnitude of the second include the nanosecond and the millisecond.


The natural units for timekeeping used by most historical societies are the day, the solar year and the lunation. Such calendars include the Sumerian, Egyptian, Chinese, Babylonian, ancient Athenian, Buddhist, Hindu, Islamic, Icelandic, Mayan, and French Republican calendars.

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

Horizontal logarithmic scale marked with units of time in the Gregorian calendar


  • The jiffy is the amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum.
  • Planck time is the time light takes to travel one Planck length. Theoretically, this is the smallest time measurement that will ever be possible. Smaller time units have no use in physics as we understand it today.
  • The TU (for Time Unit) is a unit of time defined as 1024 µs for use in engineering.
  • The Svedberg is a time unit used for sedimentation rates (usually of proteins). It is defined as 10−13 seconds (100 fs).
  • The galactic year, based on the rotation of the galaxy and usually measured in million years.[2]
  • 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 eras, which are in turn divided into periods, epochs and 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 (9 454 254 955 488 kilometers).


Many of the items in this list are rarely, if ever, used and have merely been concocted for the purpose of lists such as this. There is no limit to the number of such items that can be concocted.

Units of time
Name Length Notes
yoctoyoctosecond 10−48 s One quindecillionth of a second. The smallest unit of time.[citation needed] How about a yoctoyoctoyoctosecond?
zeptoyoctosecond 10−45 s One quattuordecillionth of a second.
Planck time 5.39×10−44 s The amount of time light takes to travel one Planck length.
attoyoctosecond 10−42 s One tredecillionth of a second.
yoctoyoctoannum 10−48 yr One quindecillionth of a year or 31.536 attoyoctoseconds.
femtoyoctosecond 10−39 s One duodecillionth of a second.
zeptoyoctoannum 10−45 yr One quattuordecillionth of a year or 31.536 femtoyoctoseconds.
picoyoctosecond 10−36 s One undecillionth of a second.
attoyoctoannum 10−42 yr One tredecillionth of a year or 31.536 picoyoctoseconds.
nanoyoctosecond 10−33 s One decillionth of a second.
femtoyoctoannum 10−39 yr One duodecillionth of a year or 31.536 nanoyoctoseconds.
microyoctosecond 10−30 s One nonillionth of a second.
picoyoctoannum 10−36 yr One undecillionth of a year or 31.536 microyoctoseconds.
milliyoctosecond 10−27 s One octillionth of a second.
nanoyoctoannum 10−33 yr One decillionth of a year or 31.536 milliyoctoseconds.
yoctosecond 10−24 s One septillionth of a second.
jiffy (physics) 3×10−24 s The amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum.
microyoctoannum 10−30 yr One nonillionth of a year or 31.536 yoctoseconds.
zeptosecond 10−21 s One sextillionth of a second. Time measurement scale of the NIST strontium atomic clock. Smallest fragment of time currently measurable is 247 zeptoseconds.[3]
milliyoctoannum 10−27 yr One octillionth of a year or 31.536 zeptoseconds.
attosecond 10−18 s One quintillionth of a second.
yoctoannum 10−24 yr One septillionth of a year or 31.536 attoseconds.
femtosecond 10−15 s One quadrillionth of a second. Pulse time on fastest lasers.
zeptoannum 10−21 yr One sextillionth of a year or 31.536 femtoseconds.
svedberg 10−13 s Time unit used for sedimentation rates (usually of proteins).
picosecond 10−12 s One trillionth of a second.
attoannum 10−18 yr One quintillionth of a year or 31.536 picoseconds.
nanosecond 10−9 s One billionth of a second. Time for molecules to fluoresce.
femtoannum 10−15 yr One quadrillionth of a year or 31.536 nanoseconds.
shake 10−8 s 10 nanoseconds, also a casual term for a short period of time.
microsecond 10−6 s One millionth of a second. Symbol is µs
picoannum 10−12 yr One trillionth of a year or 31.536 microseconds.
millisecond 10−3 s One thousandth of a second. Shortest time unit used on stopwatches.
centisecond 10−2 s One hundredth of a second.
jiffy (electronics) 1/60 s or 1/50 s Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
nanoannum 10−9 yr One billionth of a year or 31.536 milliseconds.
decisecond 10−1 s One tenth of a second.
semisemisecond / quarter second 0.25 s One quarter of a second
semisecond / half second 0.5 s One half of a second.
second s SI Base unit.
decasecond 10 s
microannum 10−6 yr One millionth of a year or 31.536 seconds.
minute 60 s
milliday 1/1000 d Also marketed as a ".beat" by the Swatch corporation.
moment 1/40 solar hour (90 s on average) Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season.[4]
hectosecond 100 s 1 minute and 40 seconds
centimilliannum 10−5 yr One hundred thousandth of a year or 315.36 seconds.
ke 864 s One hundredth of a day.
kilosecond 1000 s 16 minutes and 40 seconds
decimilliannum 10−4 yr One ten thousandth of a year or 3,153.6 seconds.
hour 60 min
milliannum 10−3 yr One thousandth of a year or 31,536 seconds.
day 24 h Longest unit used on stopwatches and countdowns.
centiannum 10−2 yr One hundredth of a year or 315,360 seconds.
week d Historically sometimes also called "sennight".
megasecond 106 s 277.777778 hours or about 1 week and 4.6 days.
fortnight weeks 14 days
lunar month 27 d h 48 min – 29 d 12 h Various definitions of lunar month exist.
month 28–31 d Occasionally calculated as 30 days.
deciannum 10−1 yr One tenth of a year or 3,153,600 seconds.
quarter and season mo
quadrimester mo
semester 18 weeks A division of the academic year.[5] Literally "six months", also used in this sense.
half year mo
lunar year 354.37 days
year 12 mo 365 or 366 d
common year 365 d 52 weeks and 1 day.
tropical year 365 d h 48 min 45.216 s[6] Average.
Gregorian year 365 d h 49 min 12 s Average.
sidereal year 365 d h min 9.7635456 s
leap year 366 d 52 weeks and d
biennium yr
triennium yr
quadrennium yr
olympiad yr
lustrum yr In early Roman times, the interval between censuses.
decade 10 yr
indiction 15 yr
gigasecond 109 s 16,666,666.6667 minutes or about 31.7 years.
jubilee 50 yr
century 100 yr
millennium 1000 yr Also called "kiloannum".
decakiloannum 104 yr Ten thousand years or ten millenniums.
terasecond 1012 s About 31,709 years.
hectokiloannum 105 yr One hundred thousand years or one hundred millenniums.
megaannum 106 yr Also called "Megayear." 1,000 millennia (plural of millennium), or 1 million years.
petasecond 1015 s About 31,709,791 years.
galactic year 2.3×108 yr[2] The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy. Around 230,000,000 years.
cosmological decade varies 10 times the length of the previous cosmological decade, with CÐ 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition.
gigaannum 109 yr Also refers to an indefinite period of time, otherwise is 1,000,000,000 years.
exasecond 1018 s About 31,709,791,983 years.
teraannum 1012 yr About 1,000,000,000,000 years.
zettasecond 1021 s About 31,709,791,983,764 years.
petaannum 1015 yr About 1,000,000,000,000,000 years.
yottasecond 1024 s About 31,709,791,983,764,584 years.
exaannum 1018 yr About 1,000,000,000,000,000,000 years.
kiloyottasecond 1027 s About 3.1709791983765E+19 years.
zettaannum 1021 yr About 1E+21 years.
megayottasecond 1030 s About 3.1709791983765E+22 years.
yottaannum 1024 yr About 1E+24 years.
gigayottasecond 1033 s About 3.1709791983765E+25 years.
kiloyottaannum 1027 yr About 1E+27 years.
terayottasecond 1036 s About 3.1709791983765E+28 years.
megayottaannum 1030 yr About 1E+30 years.
petayottasecond 1039 s About 3.1709791983765E+31 years.
gigayottaannum 1033 yr About 1E+33 years.
exayottasecond 1042 s About 3.1709791983765E+34 years.
terayottaannum 1036 yr About 1E+36 years.
zettayottasecond 1045 s About 3.1709791983765E+37 years.
petayottaannum 1039 yr About 1E+39 years.
yottayottasecond 1048 s About 3.1709791983765E+40 years.
exayottaannum 1042 yr About 1E+42 years.
zettayottaannum 1045 yr About 1E+45 years.
yottayottaannum 1048 yr About 1E+48 years. The biggest unit of time.


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 three.


  1. ^ "Definitions of the SI base units". The NIST reference on Constants, Units, and Uncertainty. National Institute of Standards and Technology. Retrieved 19 June 2021.
  2. ^ a b http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA - StarChild Question of the Month for February 2000
  3. ^ "Meet the zeptosecond, the shortest unit of time ever measured". Retrieved 2020-10-17.
  4. ^ Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. p. 190. ISBN 0-7808-0008-7.
  5. ^ "Semester". Webster's Dictionary. Retrieved 3 December 2014.
  6. ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: from Earth rotation to atomic physics. Wiley-VCH. p. 18. ISBN 3-527-40780-4., Extract of page 18