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Unit of time

Measurement unit for time
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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 SI definition is “[The second] 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 9 192 631 770 when expressed in the unit Hz, which is equal to s−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.


Main article: 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, 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.
  • The Planck time is the time light takes to travel one Planck length.
  • 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.
  • 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).


Units of time
Name Length Notes
Planck time 5.39×10−44 s The amount of time light takes to travel one Planck length.
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.
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.
attosecond 10−18 s One quintillionth of a second.
femtosecond 10−15 s One quadrillionth of a second. Pulse time on fastest lasers.
Svedberg 10−13 s Time unit used for sedimentation rates (usually of proteins).
picosecond 10−12 s One trillionth of a second.
nanosecond 10−9 s One billionth of a second. Time for molecules to fluoresce.
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
millisecond 10−3 s One thousandth of a second. Shortest time unit used on stopwatches.
jiffy (electronics) ~10-3 s Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
second 1 s SI base unit for time.
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. Also colloquially refers to a brief period of time.
hour 60 min
day 24 h Longest unit used on stopwatches and countdowns.
week d Historically sometimes also called “sennight”.
megasecond 106 s About 11.6 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.
quarantine 40 d (approximately 5.71 weeks) 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 days. From Middle English quarentine, from Italian quarantina (“forty days”), the period Venetians customarily kept ships from plague-ridden countries waiting off port, from quaranta (“forty”), from Latin quadrāgintā.
semester 18 weeks A division of the academic year. Literally “six months”, also used in this sense.
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 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
olympiad yr
lustrum yr In early Roman times, the interval between censuses.
decade 10 yr
indiction 15 yr Interval for taxation assessments (Roman Empire).
score 20 yr
gigasecond 109 s About 31.7 years.
jubilee 50 yr
century 100 yr
millennium 1000 yr Also called “kiloannum”.
terasecond 1012 s About 31,709 years.
megaannum 106 yr Also called “Megayear.” 1,000 millennia (plural of millennium), or 1 million years (in geology, abbreviated as Ma).
petasecond 1015 s About 31,709,791 years.
galactic year 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.
aeon 109 yr Also refers to an indefinite period of time, otherwise is 1,000,000,000 years.
kalpa 4.32×109 yr Used in Hindu mythology. About 4,320,000,000 years.
exasecond 1018 s About 31,709,791,983 years. Approximately 2.3 times the current age of the universe.
zettasecond 1021 s About 31,709,791,983,764 years.
yottasecond 1024 s About 31,709,791,983,764,584 years.


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.

See also


Key concepts
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and standards
Philosophy of time
Human experience
and use of time Time in science
Other fields
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International standards
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Obsolete standards
Time in physics
Archaeology and geology
Astronomical chronology
Other units of time
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