Unit of time

Table showing quantitative relationships between common units 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.
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	1 s	SI Base unit.
decasecond	10 s
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.
hectosecond	100 s	1 minute and 40 seconds
ke	864 s	14 minutes and 24 seconds
kilosecond	1000 s	16 minutes and 40 seconds
hour	60 min
decakilosecond	104 s	Ten thousand seconds or ten kiloseconds.
day	24 h	Longest unit used on stopwatches and countdowns.
hectokilosecond	105 s	One hundred thousand seconds or one hundred kiloseconds.
week	7 d	Also called "sennight".
megasecond	106 s	277.777778333333 hours or about 1 week and 4.6 days.
fortnight	2 weeks	14 days
lunar month	27 d 4 h 48 min – 29 d 12 h	Various definitions of lunar month exist.
month	28-31 days	Occasionally calculated as 30 days.
quarter and season	3 mo
quadrimester	4 mo
semester	18 weeks	A division of the academic year. Literally "six months", also used in this sense.
half year	6 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 5 h 48 min 45.216 s	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
biennium	2 yr
triennium	3 yr
quadrennium	4 yr
olympiad	4 yr
lustrum	5 yr
decade	10 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.