Unit of Measure Conversions

Time

Time Conversions

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Additional Units and Information

* Today's Gregorian calendar year remains synchronised with the solar year by using leap years
Name Symbol Value Description/Notes
aeon (eon)   1 billion years Used in astronomy
calendar year yr 365.2425 days Average length of a year in a full 400 year Gregorian calendar leap cycle
century c 100 years  
common year   365 days In the Gregorian calendar, three common years are usually followed by one leap year (see below)
decade   10 years  
fortnight   14 days;
2 weeks 		 
galactic (cosmic) year   225 million years Time the Sun takes to orbit our galaxy once
generation   25-30 years Generally accepted range
julian year aj 365.25 days Each day consists of 86,400 seconds (an ephemeris day)
leap year   366 days Leap years are those exactly divisible by 4 except century years, which are only leap years if exactly divisible by 400
lunar month - sidereal   27.321661 days Time the Moon takes to return to a given position with reference to fixed stars
(27d 7h 43min 11.5s)
lunar month - synodic   29.530589 days Average time between similar lunar phases as seen from Earth
(29d 12h 44min 2.9s with an annual ± variation up to ~7h)
lunar year   354.37 days Time taken for 12 full cycles of the phases of the moon
millennium   1,000 years  
month   30.4167 days Average length of a month in a common year
month   30.436875 days Average length of a month in a full 400 year Gregorian calendar leap cycle
month   30.5 days Average length of a month in a leap year
olympiad   4 year cycle  
quadrimester Early-year, Mid-year, Late-year 4 calendar months Quadrimesters start 1 January, 1 May and 1 September
quarter (trimester) Q1, Q2, Q3, Q4 3 calendar months Quarters start 1 January, 1 April, 1 July and 1 October
semester S1, S2 6 calendar months Semesters start 1 January and 1 July
sidereal day   23.9344696 hours Average time the Earth takes to turn once on its axis with reference to fixed stars
(23h 56min 4.09s)
sidereal year   365.256363 days Average time the Earth takes to orbit the Sun once with reference to fixed stars
(365d 6h 9min 9.76s)
solar day d 24 hours  
solar (tropical) year* at 365.2421875 days Average time the Sun takes to return to the same position in the sky as seen from Earth (365d 5h 48min 45s)

Geological Time Periods for the Earth

Name Began Duration
Pre-Cambrian 4.6 billion years ago 4.1 billion years
Cambrian 541 million years ago 56 million years
Ordovician 485 million years ago 41 million years
Silurian 444 million years ago 25 million years
Devonian 419 million years ago 60 million years
Carboniferous 359 million years ago 60 million years
Permian 299 million years ago 47 million years
Triassic 252 million years ago 51 million years
Jurassic 201 million years ago 56 million years
Cretaceous 145 million years ago 79 million years
Paleocene 66 million years ago 10 million years
Eocene 56 million years ago 22 million years
Oligocene 34 million years ago 11 million years
Miocene 23 million years ago 18 million years
Pliocene 5 million years ago 2.5 million years
Pleistocene 2.5 million years ago 2.5 million years
Holocene 11,650 years ago To present day

Useful Approximations

Approximation % Accuracy
1 week ≈ 10,000 minutes 99.2