Unit 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, $Δ ν Cs$ , 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."^[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.

Historical

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

Scientific

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.^[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 km).

Note: The parsec is not a unit of time, but a unit of length of about 30.9 trillion kilometres, despite movie references otherwise.

List

Units of time
Name	Length	Notes
Planck time	5.39×10⁻⁴⁴ s	The amount of time light takes to travel one Planck length.
quectosecond	10⁻³⁰ s	One nonillionth of a second.
rontosecond	10⁻²⁷ s	One octillionth of a second.
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 and JILA strontium atomic clock. Smallest fragment of time currently measurable is 247 zeptoseconds.^[3]
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.
centisecond	10⁻² s	One hundredth of a second.
decisecond	10⁻¹ s	One tenth of a second.
second	1 s	SI base unit for time.
decasecond	10 s	Ten seconds (one sixth of a minute)
minute	60 s
hectosecond	100 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] Also colloquially refers to a brief period of time.
kilosecond	10³ s	About 17 minutes.
hour	60 min
day	24 h	Longest unit used on stopwatches and countdowns.
week	7 d	Historically sometimes also called "sennight".
megasecond	10⁶ s	About 11.6 days.
fortnight	2 weeks	14 days
lunar month	27 d 4 h 48 min – 29 d 12 h	Various definitions of lunar month exist; sometimes also called a "lunation."
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.^[5] Literally "six months", also used in this sense.
lunar year	354.37 d
year	12 mo	365 or 366 d
common year	365 d	52 weeks and 1 day.
tropical year	365 d 5 h 48 min 45.216 s^[6]	Average.
Gregorian year	365 d 5 h 49 min 12 s	Average.
sidereal year	365 d 6 h 9 min 9.7635456 s
leap year	366 d	52 weeks and 2 d
olympiad	4 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.^[7] It is also used in reference to the four-year interval between leap years, for example when wishing friends and family a "happy quadrennium" on February 29.^{[citation needed]}
lustrum	5 yr	In early Roman times, the interval between censuses.
decade	10 yr
indiction	15 yr	Interval for taxation assessments (Roman Empire).
gigasecond	10⁹ s	About 31.7 years.
jubilee	50 yr
century	100 yr
millennium	1000 yr	Also called "kiloannum".
Age	2,148 and two thirds of a year	a unit used in astrology, each of them represent a star sign
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	2.3×10⁸ yr	The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy (approx 230,000,000 years^[2]).
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.
eon	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 31,709,791,983,764,586 years.
ronnasecond	10²⁷ s	About 31,709,791,983,764,586,504 years.
quettasecond	10³⁰ s	About 31,709,791,983,764,586,504,312 years.

Interrelation

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.