Unit 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 base unit of time in the International System of Units 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 duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-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 is not 28 days but 28.3 days. The year, defined in the Gregorian calendar as days has to be adjusted with leap days and leap seconds. Consequently, these units are now all defined 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.

Scientific time units

Note: The light-year is not a unit of time, but a unit of length of about 9.5 petametres.


UnitLength, Duration and SizeNotes
Planck time unitThe 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.
jiffy The amount of time light takes to travel one fermi in a vacuum.
zeptosecondTime measurement scale of the NIST strontium atomic clock. Smallest fragment of time currently measurable is 850 zeptoseconds.
femtosecondPulse time on fastest lasers.
SvedbergTime unit used for sedimentation rates.
nanosecondTime for molecules to fluoresce.
shake10 nanoseconds, also a casual term for a short period of time.
microsecondSymbol is µs
millisecondShortest time unit used on stopwatches.
jiffy or Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
secondSI Base unit.
moment Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season.
hectosecond1 minute and 40 seconds
keUsually calculated as 15 minutes, similar to "quarter" as in "a quarter past six".
kilosecond16 minutes and 40 seconds
dayLongest unit used on stopwatches and countdowns.
weekAlso called "sennight".
megasecond277.777778333333 hours or about 1 week and 4.6 days.
fortnight14 days
lunar monthVarious definitions of lunar month exist.
monthOccasionally calculated as 30 days.
quarter and season
semesterA division of the academic year. Literally "six months", also used in this sense.
common year52 weeks and 1 day.
tropical yearAverage.
Gregorian yearAverage.
sidereal year-
leap year and
gigasecond16,666,666.6667 minutes or About 31.7 years.
millenniumAlso called "kiloannum".
terasecondabout 31,700 years.
Also called "Megayear." About 1,000 millennia, or 1 million years.
petasecondAbout 31,700,000 years or 380,399,583.12 months
galactic yearThe amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time.
cosmological decadevaries10 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.
aeonAlso spelled "eon". Also refers to an indefinite period of time.
exasecondAbout 31,700,000,000 years or 380,399,583,123.74 months
zettasecondAbout 31.7 trillion years or 3,803,995,983,123,744.56 months
yottasecondAbout 31.7 quadrillion years or 380,399,583,123,744,510 months

Units of time interrelated

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.