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Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future. Time is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as the fourth dimension, along with the three spatial dimensions.

Time has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars. Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems. Two contrasting viewpoints on time divide prominent philosophers. One view is that time is part of the fundamental structure of the universe—a dimension independent of events, in which events occur in sequence. Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time. The opposing view is that time does not refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz and Immanuel Kant, holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.

Units of time
Unit Length, duration and size Notes
instant varies loosely speaking, zero time (colloquially the term may be used in other ways)
Planck time unit 5.39 x 10−44 s The duration light takes to travel one Planck length. Theorized to be the smallest duration measurement that will ever be possible, roughly 10−43 seconds.
yoctosecond 10−24 s
jiffy (quantum physics) about 3 × 10−24 The duration light takes to travel one fermi (10−15 m, about the size of a nucleon) in a vacuum.
zeptosecond 10−21 s
attosecond 10−18 s shortest duration now measurable
femtosecond 10−15 s pulse duration on fastest lasers
picosecond 10−12 s
nanosecond 10−9 s duration for molecules to fluoresce
shake 10−8 s 10 nanoseconds. Also a casual term for a short duration.
microsecond 10−6 s
millisecond 0.001 s shortest duration unit used on stopwatches
centisecond 0.01 s used on some stopwatches
jiffy (electronics) ~1/50 s to 1/60 s Used to measure the duration between alternating power cycles. Also a casual term for a short duration
decisecond 0.1 s Used on some stopwatches
second 1 s SI base unit
decasecond 10 seconds
half a minute 30 seconds
minute 60 seconds
moment (historical) 1/40th of an hour? (90 seconds) used by Medieval Western European computists.
hectosecond 100 seconds 1 minute and 40 seconds
5 minutes 300 seconds All the numbers on an analog clock are 5 minutes apart
centiday 864 seconds traditional Chinese unit of decimal time duration, usually 1/100 of a day, apart by 100 ke(scales) on the sundial and the ruler of water clock, i.e. 14 minutes and 24 seconds. (Nearly 1/4 of an hour. Similar to the English word "quarter" as in "a quarter past six", i.e. 6:15)
kilosecond 1,000 seconds 16 minutes and 40 seconds
point 24 minutes Traditional Chinese time unit, usually 1/60 of a day.
hour 60 minutes
dualhour 2 hours Traditional time unit, 1/12 of a day. apart by 12 shi on the sundial.
deciday 2.4 hours Traditional unit of decimal time duration, 1/10 of a day. apart by geng
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 14 days 2 weeks (more common in Great Britain)
lunar month 27.2–29.5 days Various definitions of lunar month exist.
February 28–29 days This month gets an extra day in a leap year
common month 30–31 days Often 30 days for financial and other calculations.
quarter and season 3 months The duration of any of the four calendar seasons; winter, spring, summer and autumn.
year 12 months
common year 365 days 52 weeks + 1 day
tropical year 365.24219 days average
Gregorian year 365.2425 days average
Julian year 365.25 days
sidereal year 365.256363004 days
leap year 366 days 52 weeks + 2 days
biennium 2 years A unit of time duration commonly used by legislatures
triennium 3 years
Olympiad 4-year cycle
lustrum 5 years
decade 10 years
Indiction 15-year cycle
generation varies about 17–36 years for humans, but some are more extreme
gigasecond 1,000,000,000 seconds About 31.7 years
jubilee 50 years
Lifespan 85 or 82 years how long (on average) people live
century 100 years
millennium 1,000 years also called "kiloannum"
terasecond 1012 seconds About 31,700 years
megaannum 1,000,000 years 1 million years
age varies on the geological timescale, some millions of years
epoch varies on the geological timescale, tens of millions of years
petasecond 1015 seconds About 31.7 million years
era varies on the geological timescale, several hundred millions of years
galactic year Approximately 230 million years The duration it takes the Sun to orbit the center of the Milky Way galaxy once.
eon varies on the geological timescale, 500 million years or more. Also "an indefinite and very long period of time".
gigaannum 1,000,000,000 years A billion years (109)
Our star's lifespan 12,000,000,000 years Is how long the sun will live
exasecond 1018 seconds roughly 31.7 x 109 years, more than twice the age of the universe (on current estimates)
teraannum 1,000,000,000,000 years A trillion years (1012)
zettasecond 1021 seconds About 31.7 x 1012 years
petaannum 1,000,000,000,000,000 years A quadrillion years (1015)
yottasecond 1024 seconds About 31.7 x 1015 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.
System Description UT1 UTC TT TAI GPS
UT1 Mean Solar Time UT1 UTC = UT1 - DUT1 TT = UT1 + 32.184 s + LS - DUT1 TAI = UT1 - DUT1 + LS GPS = UT1 - DUT1 + LS - 19 s
UTC Civil Time UT1 = UTC + DUT1 UTC TT = UTC + 32.184 s + LS TAI = UTC + LS GPS = UTC + LS - 19 s
TT Terrestrial (Ephemeris) Time UT1 = TT - 32.184 s - LS + DUT1 UTC = TT - 32.184 s - LS TT TAI = TT - 32.184 s GPS = TT - 51.184 s
TAI Atomic Time UT1 = TAI + DUT1 - LS UTC = TAI - LS TT = TAI + 32.184 s TAI GPS = TAI - 19 s
GPS GPS Time UT1 = GPS + DUT1 - LS + 19 s UTC = GPS - LS + 19 s TT = GPS + 51.184 s TAI = GPS + 19 s GPS

  • instant as an object—one point on the time axes. Being an object, it has no value;
  • time interval as an object—part of the time axes limited by two instants. Being an object, it has no value;
  • date as a quantity characterizing an instant. As a quantity, it has a value which may be expressed in a variety of ways, for example "2014-04-26T09:42:36,75" in ISO standard format, or more colloquially such as "today, 9:42 a.m.";
  • duration as a quantity characterizing a time interval. As a quantity, it has a value, such as a number of minutes, or may be described in terms of the quantities (such as times and dates) of its beginning and end.
  • Models including the Hartle–Hawking boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.
  • Brane cosmology models in which inflation is due to the movement of branes in string theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically.
  • Chaotic inflation, in which inflation events start here and there in a random quantum-gravity foam, each leading to a bubble universe expanding from its own big bang.
  • Charlie Gere, (2005) Art, Time and Technology: Histories of the Disappearing Body, Berg
  • Craig Callendar, Introducing Time, Icon Books, 2010,
  • Benjamin Gal-Or, Cosmology, Physics and Philosophy, Springer Verlag, 1981, 1983, 1987, , .
  • Roberto Mangabeira Unger and Lee Smolin, The Singular Universe and the Reality of Time, Cambridge University Press, 2014, .


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