A unit of time or time unit is any particular time intravel, used as a
standard way of measuring or expressing duration. The base unit of
time in the
International System of Units
The duration of 7009919263177000000♠9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
Historically units of time were defined by the movements of astronomical objects.
These units do not have a consistent relationship with each other and
require intercalation. For example, the year cannot be divided into 12
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
1 Historical 2 Scientific time units 3 List 4 Units of time interrelated 5 References
Horizontal logarithmic scale marked with units of time in the Gregorian calendar
Scientific time units
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
The TU (for
Note: The light-year is not a unit of time, but a unit of length of about 9 trillion kilometres (9,454,254,955,488 kilometres). List
Units of time
Unit Length, Duration and Size Notes
Planck time unit 5.39 x 10−44 s The amount of 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.
yoctosecond 10−24 s
jiffy (physics) 3 × 10−24s The amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum.
attosecond 10−18 s
femtosecond 10−15 s Pulse time on fastest lasers.
picosecond 10−12 s
shake 10−8 s 10 nanoseconds, also a casual term for a short period of time.
microsecond 10−6 s Symbol is µs
millisecond 0.001 s Shortest time unit used on stopwatches.
jiffy (electronics) 1/60s to 1/50s Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
second 1 sec SI Base unit.
minute 60 seconds
moment 1/40th of an hour (90 seconds) Medieval unit of time used by astronomers to compute astronomical movements.
ke 14 minutes and 24 seconds Usually calculated as 15 minutes, similar to "quarter" as in "a quarter past six" (6:15).
kilosecond 1,000 seconds 16 minutes and 40 seconds.
hour 60 minutes
day 24 hours Longest unit used on stopwatches and countdowns.
week 7 days Also called "sennight".
megasecond 1,000,000 seconds About 11.6 days.
fortnight 2 weeks 14 days
lunar month 27 days 4 hours 48 minutes–29 days 12 hours Various definitions of lunar month exist.
month 28–31 days Occasionally calculated as 30 days.
quarter and season 3 months
semester an 18-week division of the academic year Literally "six months", also used in this sense.
year 12 months or 365 days
common year 365 days 52 weeks and 1 day.
tropical year 365 days and 5:48:45.216 hours Average.
Gregorian year 365 days and 5:49:12 hours Average.
sidereal year 365 days and 6:09:09.7635456 hours
leap year 366 days 52 weeks and 2 days.
biennium 2 years
triennium 3 years
quadrennium 4 years
olympiad 4 year cycle 48 months, 1,461 days, 35,064 hours, 2,103,840 minutes, 126,230,400 seconds.
lustrum 5 years
decade 10 years
indiction 15 year cycle
score 20 years
gigasecond 1,000,000,000 seconds About 31.7 years.
jubilee 50 years
century 100 years
millennium 1,000 years Also called "kiloannum".
terasecond 1 trillion seconds About 31,700 years.
Megannum 1,000,000 (106) years Also called "Megayear." About 1,000 millennia (plural of millennium), or 1 million years.
petasecond 1015 seconds About 31,700,000 years
galactic year Approximately 230 million years The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time.
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 1,000,000,000 years or an indefinite period of time Also spelled "eon"
exasecond 1018 seconds About 31,700,000,000 years
zettasecond 1021 seconds About 31.7 trillion years
yottasecond 1024 seconds About 31.7 x 1015 years
Units of time interrelated
Flowchart illustrating the major units of time
All of the important units of time can be interrelated. The key units are the second, defined in terms of an atomic process; the day, an integral multiple of seconds; and the year, usually 365.25 days. Most of the other units used are multiples or divisions of these three. The graphic also shows the three heavenly bodies whose orbital parameters relate to the units of time. References
^ "Definitions of the SI base units". The NIST reference on Constants,
Units, and Uncertainty. National Institute of Standards and
Technology. Retrieved 4 March 2016.
^ a b
NASA - StarChild Question of the
v t e
history deep time
Measurement and standards
UTC UT TAI Unit of time Planck time Second Minute Hour Day Week Month Season Year Decade Century Millennium Tropical year Sidereal year Samvatsara
Gregorian Julian Hebrew Islamic Lunar Solar Hijri Mayan Intercalation Leap second Leap year
marine sundial sundial markup schema watch water-based
Philosophy of time
A-series and B-series B-theory of time Causality Duration Endurantism Eternal return Eternalism Event Multiple time dimensions Perdurantism Presentism Static interpretation of time Temporal finitism Temporal parts The Unreality of Time
Human experience and use of time
age chron eon epoch era period
Geochronology Geological history of Earth
Absolute time and space
Arrow of time
Theory of relativity
other subject areas
Dating methodologies in archaeology
v t e
Chronometry Orders of magnitude Metrology
Coordinated Universal Time
International Earth Rotation and Reference Systems Service
International Atomic Time
Barycentric Coordinate Time
Barycentric Dynamical Time
Daylight saving time
Geocentric Coordinate Time
International Date Line
Ephemeris time Greenwich Mean Time Prime meridian
Absolute time and space
Discrete time and continuous time
Theory of relativity
Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Intercalation Islamic Julian Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names
Archaeology and geology
Chronological dating Geologic time scale International Commission on Stratigraphy
Galactic year Nuclear timescale Precession Sidereal time
Other units of time
Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium