Time - Definitions and Standards

Definitions and Standards

Units of time

Unit	Size	Notes
yoctosecond	10−24 s
zeptosecond	10−21 s
attosecond	10−18 s	shortest time uncertainty in present measurements
femtosecond	10−15 s	pulse time of ultrafast lasers (100 as = 0.1 fs)
picosecond	10−12 s
nanosecond	10−9 s	time for molecules to fluoresce
microsecond	10−6 s
millisecond	0.001 s
second	1 s	SI base unit
minute	60 seconds
hour	60 minutes
day	24 hours
week	7 days	Also called sennight
fortnight	14 days	2 weeks
lunar month	27.2–29.5 days	Various definitions exist.
month	28–31 days
quarter	3 months
year	12 months
common year	365 days	52 weeks + 1-day
leap year	366 days	52 weeks + 2 days
tropical year	365.24219 days	average
Gregorian year	365.2425 days	average
Olympiad	4-year cycle
lustrum	5 years	Also called pentad
decade	10 years
Indiction	15-year cycle
generation	17–35 years	approximate
jubilee (Biblical)	50 years
century	100 years
millennium	1,000 years
exasecond	1018 s
cosmological decade	varies
cosmological century	varies

See also: Time standard and Orders of magnitude (time)

The SI base unit for time is the SI second. From the second, larger units such as the minute, hour and day are defined, though they are "non-SI" units because they do not use the decimal system, and also because of the occasional need for a leap second. They are, however, officially accepted for use with the International System. There are no fixed ratios between seconds and months or years as months and years have significant variations in length.

The official SI definition of the second is as follows:

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

At its 1997 meeting, the CIPM affirmed that this definition refers to a caesium atom in its ground state at a temperature of 0 K. Previous to 1967, the second was defined as:

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.

The current definition of the second, coupled with the current definition of the metre, is based on the special theory of relativity, which affirms our space-time to be a Minkowski space.