Unit of time

Visualisation of units of time from one second to one average year of the Gregorian calendar

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 definition, from the National Institute of Standards and Technology is:

The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.[1]

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 Gregorian calendar as 365.25 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.


Main article: History of calendars

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, 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 time units

The light-year is not a unit of time, but a unit of length about 9 trillion kilometres.


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.[3] 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.
zeptosecond 10−21 s Time measurement scale of the NIST strontium atomic clock
attosecond 10−18 s Shortest time now measurable.
femtosecond 10−15 s Pulse time on fastest lasers.
Svedberg 10−13 s Time unit used for sedimentation rates (usually of proteins).
picosecond 10−12 s
nanosecond 10−9 s Time for molecules to fluoresce.
shake 10−8 s 10 nanoseconds, also a casual term for a short period of time.
microsecond 10−6 sSymbol 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.[3]
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.
semester an 18-week division of the academic year[4] Literally "six months", also used in this sense.
quarter and season 3 months
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[5] Average.
Gregorian year 365 days and 5:49:12 hours[6] Average.
sidereal year 365 days and 6:09:09.7635456 hours
leap year 366 days 52 weeks and 2 days.
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
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.
galactic year Approximately 230 million years[2]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"

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.


  1. "Definitions of the SI base units". The NIST reference on Constants, Units, and Uncertainty. National Institute of Standards and Technology. Retrieved 4 March 2016.
  2. 1 2 http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA - StarChild Question of the Month for February 2000
  3. Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. p. 190. ISBN 0-7808-0008-7.
  4. . Webster's Dictionary http://www.merriam-webster.com/dictionary/semester. Retrieved 3 December 2014. Missing or empty |title= (help)
  5. 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
  6. Jones, Floyd Nolen (2005). The Chronology Of The Old Testament (15th ed.). New Leaf Publishing Group. p. 287. ISBN 0-89051-416-X., Extract of page 287
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