# 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 (SI) and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom.

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).

## List

Units of time
Unit Length, Duration and Size 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.[1]
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
centimillisecond 10−5 s One hundred thousandth of a second or one hundredth of a millisecond.
decimillisecond 10−4 s One ten-thousandth of a second or one tenth of a millisecond.
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.
decisecond 10−1 s One tenth of a second.
quadrisecond / semisemisecond 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
half a minute 30 s It’s known by math.
minute 60 s
milliday 1/1000 d Also marketed as a ".beat" by the Swatch corporation; Milliday is a more accurate name.
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.[2]
hectosecond 100 s 1 minute and 40 seconds
ke 864 s 14 minutes and 24 seconds
kilosecond 1000 s 16 minutes and 40 seconds
half an hour 30 min It’s known by math.
hour 60 min
decakilosecond 104 s Ten thousand seconds or ten kiloseconds.
Half an day 12 h Its known by math.
day 24 h Longest unit used on stopwatches and countdowns.
hectokilosecond 105 s One hundred thousand seconds or one hundred kiloseconds.
week d Also called "sennight".
megasecond 106 s 277.777778333333 hours or about 1 week and 4.6 days.
fortnight 14 days
lunar month 27 d h 48 min – 29 d 12 h Various definitions of lunar month exist.
month 28-31 days Occasionally calculated as 30 days.
quarter and season mo
semester 18 weeks A division of the academic year.[3] 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[4] 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
lustrum 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".
terasecond 1012 s About 31,709 years.
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 The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time. 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.
zettasecond 1021 s About 31,709,791,983,764 years.
yottasecond 1024 s About 31,709,791,983,764,584 years.

| - | The Age Of The Universe || 13700000000 yr || Alternatively, it’s roughly 4.35 yottaseconds |}

## References

1. "Meet the zeptosecond, the shortest unit of time ever measured". Retrieved 2020-10-17.
2. Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. p. 190. ISBN 0-7808-0008-7.
3. "Semester". Webster's Dictionary. Retrieved 3 December 2014.
4. 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