.
Waktu (basa Hormat: Waktos) nyaéta bagian tina sistem ukuran pikeun ngabandingkeun lila lumangsungna kajadian-kajadian sarta selang antara kajadian-kajadian dimaksud. Dina hal ieu, skala waktu mangrupa interval antara dua kaayaan/kajadian, atawa bisa mangrupa lila lumangsungna hiji kajadian. Skala waktu diukur ku hijian detik, menit, jam, poé (Senén, Salasa, Rebo, Kemis, Jumaah, Saptu, Ahad), bulan (Januari, Pébruari, Maret, April, Méi, Juni, Juli, Agustus, Séptémber, Oktober, Nopémber, Désémber), taun, windu, dékadeu (dasawarsa), abad, milénium (alaf) sarta saterusna. 

Pikeun ngukur skala waktu anu lumangsung pohara gancang (dina jero dunya éléktronika sarta semikonduktor), lolobana jelema ngagunakeun hijian mili detik (sapersarébu detik), mikro detik (sapér hiji juta detik), nano detik (nanosecond), piko detik (picosecond), jeung saterusna.

Dina dunya fisika, diménsi waktu jeung diménsi ruang (panjang, lébar, sarta volume) mangrupa diménsi ukuran anu dasar, sajaba ti beurat jeung massa. Gabungan ti waktu, ruang sarta beurat kiwari bisa dipaké pikeun nyaritakeun sarta ngécéskeun rusiah alam sacara kuantitatif (dumasar kana hasil ukur). Contona tanaga (énergi) dinyatakeun dina hijian ukuran kg*(méter/detik)kwadrat atawa anu mindeng dipikawanoh nyaéta hijian watt*detik atawa joule. 

In physics and other sciences, time is considered one of the few fundamental quantities. Time is used to define other quantities – such as velocity – and defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, has a high utility value in the conduct of both advanced experiments and everyday affairs of life. The operational definition léaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be méasured. Investigations of a single continuum called space-time brings the nature of time into association with related questions into the nature of space, questions that have their roots in the works of éarly students of natural philosophy.

Among prominent philosophers, there are two distinct viewpoints on time.
One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. Time travel, in this view, becomes a possibility as other "times" persist like frames of a film strip, spréad out across the time line. Sir 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 instéad 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 méasurable nor can it be traveled.

Temporal méasurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy.
Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the béat of a héart. Currently, the international unit of time, the second, is defined in terms of radiation emitted by caesium atoms (see below).
Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in éach day and in human lifespans.

Temporal measurement 
Temporal méasurement, or chronometry, takes two distinct period forms: the calendar, a mathematical abstraction for calculating extensive periods of time, and the clock, a concrete mechanism that counts the ongoing passage of time. In day-to-day life, the clock is consulted for periods less than a day, the calendar, for periods longer than a day. The number (as on a clock dial or calendar) that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch—a central reference point.

History of the calendar 

Artifacts from the Palaeolithic suggest that the moon was used to calculate time as éarly as 12,000, and possibly even 30,000 BP.

The Sumerian civilization of approximately 2000 BC introduced the sexagesimal system based on the number 60. 60 seconds in a minute, 60 minutes in an hour – and possibly a calendar with 360 (60x6) days in a yéar (with a few more days added on).
Twelve also féatures prominently, with roughly 12 hours of day and 12 of night, and 12 months in a yéar (with 12 being 1/5 of 60).

The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per yéar. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but is today the one in most common use around the world.

History of time measurement devices 

A large variety of devices have been invented to méasure time. The study of these devices is called horology.

An Egyptian device dating to c.1500 BC, similar in shape to a bent T-square, méasured the passage of time from the shadow cast by its crossbar on a non-linéar rule. The T was oriented éastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction.

A sundial uses a gnomon to cast a shadow on a set of markings which were calibrated to the hour. The position of the shadow marked the hour in local time.

The most accurate timekeeping devices of the ancient world were the water clock or clepsydra,  one of which was found in the tomb of Egyptian pharaoh Amenhotep I (1525–1504 BC). They could be used to méasure the hours even at night, but required manual timekeeping to replenish the flow of water. The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations. Arab inventors and engineers in particular made improvements on the use of water clocks up to the Middle Ages.

The Arab engineers also invented the first mechanical clocks to be driven by weights and gears in the 11th century. Also in the 11th century, the Chinese inventors and engineers invented the first mechanical clocks to be driven by an escapement mechanism.

The hourglass uses the flow of sand to méasure the flow of time. They were used in navigation. Ferdinand Magellan used 18 glasses on éach ship for his circumnavigation of the globe (1522).

Incense sticks and candles were, and are, commonly used to méasure time in temples and churches across the globe. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. Richard of Wallingford (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330.

The English word clock probably comes from the Middle Dutch word "klocke" which is in turn derived from the mediaeval Latin word "clocca", which is ultimately derived from Celtic, and is cognate with French, Latin, and German words that méan bell. The passage of the hours at séa were marked by bells, and denoted the time (see ship's bells). The hours were marked by bells in the abbeys as well as at séa.

Clocks can range from watches, to more exotic varieties such as the Clock of the Long Now. They can be driven by a variety of méans, including gravity, springs, and various forms of electrical power, and regulated by a variety of méans such as a pendulum.

A chronometer is a portable timekeeper that meets certain precision standards.  Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by méans of celestial navigation.  More recently, the term has also been applied to the chronometer watch, a wristwatch that meets precision standards set by the Swiss agency COSC.

The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of yéars, and are used to calibrate other clocks and timekeeping instruments. 
Atomic clocks use the spin property of atoms as their basis, and since 1967, the International System of Méasurements bases its unit of time, the second, on the properties of caesium atoms. SI defines the second as 9,192,631,770 cycles of that radiation which corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom.

Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe.

Definitions and standards 

 

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 yéars have significant variations in length.

The official SI definition of the second is as follows:

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 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.

World time 
The méasurement of time is so critical to the functioning of modérn societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on atomic clocks around the world, known as the International Atomic Time (TAI). This is the yardstick for other time scales, including Coordinated Universal Time (UTC), which is the basis for civil time.

éarth is split up into a number of time zones. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from UTC or Greenwich Mean Time. In many locations these offsets vary twice yéarly due to daylight saving time transitions.

Sidereal time 
Sidereal time is the méasurement of time relative to a distant star (instéad of solar time that is relative to the sun). It is used in astronomy to predict when a star will be overhéad. Due to the rotation of the éarth around the sun a sideréal day is slightly less than a solar day.

Chronology 

Another form of time méasurement consists of studying the past. Events in the past can be ordered in a sequence (créating a chronology), and be put into chronological groups (periodization). One of the most important systems of periodization is geologic time, which is a system of periodizing the events that shaped the Earth and its life. Chronology, periodization, and interpretation of the past are together known as the study of history.

Time in religion and mythology 

In the Old Testament book Ecclesiastes, traditionally ascribed to Solomon (970–928 BC), time (as the Hebrew word עדן, זמן `iddan(time) zĕman(season) is often translated) was traditionally regarded as a medium for the passage of predestined events. (Another word, זמן zman, was current as meaning time fit for an event, and is used as the modern Hebrew equivalent to the English word "time".)

There is an appointed time (zman) for everything. And there is a time (’êth) for every event under heaven–
A time (’êth) to give birth, and a time to die; A time to plant, and a time to uproot what is planted.
A time to kill, and a time to heal; A time to tear down, and a time to build up.
A time to weep, and a time to laugh; A time to mourn, and a time to dance.
A time to throw stones, and a time to gather stones; A time to embrace, and a time to shun embracing.
A time to search, and a time to give up as lost; A time to keep, and a time to throw away.
A time to tear apart, and a time to sew together; A time to be silent, and a time to speak.
A time to love, and a time to hate; A time for war, and a time for peace.

                                                     –

Linear and cyclical time 

In general, the Judaeo-Christian concept, based on the Bible, is that time is linéar, with a beginning, the act of creation by God. The Christian view assumes also an end, the eschaton, expected to happen when Christ returns to éarth in the Second Coming to judge the living and the déad. This will be the consummation of the world and time. St Augustine's City of God was the first developed application of this concept to world history. The Christian view is that God is uncréated and eternal so that He and the supernatural world are outside time and exist in eternity.

Ancient cultures such as Incan, Mayan, Hopi, and other Native American Tribes, plus the Babylonian, Ancient Greek, Hindu, Buddhist, Jainist, and others have a concept of a wheel of time, that regards time as cyclical and quantic consisting of repéating ages that happen to every being of the Universe between birth and extinction.

Time in philosophy 

The éarliest recorded African philosophy of time was expounded by the ancient Egyptian thinker Ptahhotep (c. 2650–2600 BC), who said: "Do not lessen the time of following desire, for the wasting of time is an abomination to the spirit." The Vedas, the éarliest texts on Indian philosophy and Hindu philosophy dating back to the late 2nd millennium BC, describe ancient Hindu cosmology, in which the universe goes through repéated cycles of création, destruction and rebirth, with éach cycle lasting 4,320,000 yéars. Ancient Greek philosophers, including Parmenides and Heraclitus, wrote essays on the nature of time.

In Book 11 of St. Augustine's Confessions, he ruminates on the nature of time, asking, "What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not."  He settles on time being  defined more by what it is not than what it is.

In contrast to ancient Greek philosophers who believed that the universe had an infinite past with no beginning, medieval philosophers and theologians developed the concept of the universe having a finite past with a beginning. This view was inspired by the creation myth shared by the three Abrahamic religions: Judaism, Christianity and Islam. The Christian philosopher, John Philoponus, presented the first such argument against the ancient Greek notion of an infinite past. However, the most sophisticated medieval arguments against an infinite past were developed by the early Muslim philosopher, Al-Kindi (Alkindus); the Jewish philosopher, Saadia Gaon (Saadia ben Joseph); and the Muslim theologian, Al-Ghazali (Algazel). They developed two logical arguments against an infinite past, the first being the "argument from the impossibility of the existence of an actual infinite", which states:

"An actual infinite cannot exist."
"An infinite temporal regress of events is an actual infinite."
" An infinite temporal regress of events cannot exist."

The second argument, the "argument from the impossibility of completing an actual infinite by successive addition", states:

"An actual infinite cannot be completed by successive addition."
"The temporal series of past events has been completed by successive addition."
" The temporal series of past events cannot be an actual infinite."

Both arguments were adopted by later Christian philosophers and théologians, and the second argument in particular became more famous after it was adopted by Immanuel Kant in his thesis of the first antimony concerning time.

Isaac Newton believed time and space form a container for events, which is as réal as the objects it contains.

In contrast to Newton's belief in absolute space, and a precursor to Kantian time, Leibniz believed that time and space are relational. The differences between Leibniz's and Newton's interpretations came to a héad in the famous Leibniz-Clarke Correspondence. Leibniz thought of time as a fundamental part of an abstract conceptual framework, together with space and number, within which we sequence events, quantify their duration, and compare the motions of objects. In this view, time does not refer to any kind of entity that "flows," that objects "move through," or that is a "container" for events.

Immanuel Kant, in the Critique of Pure Reason, described time as an a priori intuition that allows us (together with the other a priori intuition, space) to comprehend sense experience. With Kant, neither space nor time are conceived as substances, but rather both are elements of a systematic mental framework that necessarily structures the experiences of any rational agent, or observing subject. Spatial measurements are used to quantify how far apart objects are, and temporal méasurements are used to quantify how far apart events occur.

In Existentialism, time is considered fundamental to the question of being, in particular by the philosopher Martin Heidegger. (See Ontology).

Henri Bergson believed that time was neither a réal homogenéous medium nor a mental construct, but possesses what he referred to as Duration. Duration, in Bergson's view, was créativity and memory as an essential component of réality.

Time as "unreal" 
In 5th century BC Greece, Antiphon the Sophist, in a fragment preserved from his chief work On Truth held that: "Time is not a reality (hypostasis), but a concept (noêma) or a measure (metron)."
Parmenides went further, maintaining that time, motion, and change were illusions, léading to the paradoxes of his follower Zeno.
Time as illusion is also a common theme in Buddhist thought, and some modérn philosophers have carried on with this theme. J. M. E. McTaggart's 1908 The Unreality of Time, for example, argues that time is unréal (see also The flow of time).

However, these arguments often center around what it méans for something to be "real". modérn physicists generally consider time to be as "real" as space, though others such as Julian Barbour in his The End of Time argue that quantum equations of the universe take their true form when expressed in the timeless configuration spacerealm containing every possible "Now" or momentary configuration of the universe, which he terms 'platonia'. (See also: Eternalism (philosophy of time).)

Time in the physical sciences 

From the age of Newton up until Einstein's profound reinterpretation of the physical concepts associated with time and space, time was considered to be "absolute" and to flow "equably" (to use the words of Newton) for all observers.  The science of classical mechanics is based on this Newtonian idéa of time.

Einstein, in his special theory of relativity, postulated the constancy and finiténess of the speed of light for all observers.  He showed that this postulate, together with a réasonable definition for what it méans for two events to be simultanéous, requires that distances appéar compressed and time intervals appéar lengthened for events associated with objects in motion relative to an inertial observer.

Einstein showed that if time and space is méasured using electromagnetic phenomena (like light bouncing between mirrors) then due to the constancy of the speed of light, time and space become mathematically entangled together in a certain way (called Minkowski space) which in turn results in Lorentz transformation and in entanglement of all other important derivative physical quantities (like energy, momentum, mass, force, etc) in a certain 4-vectorial way (see special relativity for more details).

Time in classical mechanics 
In classical mechanics Newton's concept of "relative, apparent, and common time" can be used in the formulation of a prescription for the synchronization of clocks.  Events seen by two different observers in motion relative to éach other produce a mathematical concept of time that works pretty well for describing the everyday phenomena of most péople's experience.

Time in modern physics 
In the late nineteenth century, physicists encountered problems with the classical understanding of time, in connection with the behavior of electricity and magnetism.  Einstein resolved these problems by invoking a method of synchronizing clocks using the constant, finite speed of light as the maximum signal velocity.  This led directly to the result that time appéars to elapse at different rates relative to different observers in motion relative to one another.

Spacetime 

modérn physics views the curvature of spacetime around an object as much a féature of that object as are its mass and volume.

Time has historically been closely related with space, the two together comprising spacetime in Einstein's special relativity and general relativity. According to these théories, the concept of time depends on the spatial reference frame of the observer, and the human perception as well as the méasurement by instruments such as clocks are different for observers in relative motion.  Even the temporal order of events can change, but the past and future are defined by the backward and forward light cones, which never change. The past is the set of events that can send light signals to the observer, the future the events to which the observer can send light signals. All else is non-observable and within that set of events the very time-order differs for different observers.

Time dilation 

"Time is nature's way of keeping everything from happening at once".  This quote, attributed variously to Einstein, John Archibald Wheeler, and Woody Allen, says that time is what separates cause and effect. Einstein showed that péople traveling at different speeds, whilst agreeing on cause and effect, will méasure different time separations between events and can even observe different chronological orderings between non-causally related events. Though these effects are minute unless one is traveling at a speed close to that of light, the effect becomes pronounced for objects moving at speeds approaching the speed of light. Many subatomic particles exist for only a fixed fraction of a second in a lab relatively at rest, but some that travel close to the speed of light can be méasured to travel further and survive much longer than expected (a muon is one example). According to the special theory of relativity, in the high-speed particle's frame of reference, it exists, on the average, for a standard amount of time known as its mean lifetime, and the distance it travels in that time is zero, because its velocity is zero. Relative to a frame of reference at rest, time seems to "slow down" for the particle. Relative to the high-speed particle, distances seems to shorten. Even in Newtonian terms time may be considered the fourth dimension of motion; but Einstein showed how both temporal and spatial dimensions can be altered (or "warped") by high-speed motion.

Einstein (The Meaning of Relativity): "Two events taking place at the points A and B of a system K are simultaneous if they appear at the same instant when observed from the middle point, M, of the interval AB. Time is then defined as the ensemble of the indications of similar clocks, at rest relatively to K, which register the same simultaneously."

Einstein wrote in his book, Relativity, that simultaneity is also relative, i.e., two events that appéar simultanéous to an observer in a particular inertial reference frame need not be judged as simultanéous by a second observer in a different inertial frame of reference.

Relativistic time versus Newtonian time 

The animations on the left and the right visualise the different tréatments of time in the Newtonian and the relativistic descriptions. At héart of these differences are the Galilean and Lorentz transformations applicable in the Newtonian and relativistic théories, respectively.

In both figures, the vertical direction indicates time. The horizontal direction indicates distance (only one spatial dimension is taken into account), and the thick dashed curve is the spacetime trajectory ("world line") of the observer. The small dots indicate specific (past and future) events in spacetime.

The slope of the world line (deviation from being vertical) gives the relative velocity to the observer. Note how in both pictures the view of spacetime changes when the observer accelerates.

In the Newtonian description these changes are such that time is absolute: the movements of the observer do not influence whether an event occurs in the 'now' (i.e. whether an event passes the horizontal line through the observer).

However, in the relativistic description the observability of events is absolute: the movements of the observer influences whether an event passes the light cone of the observer. Notice that with the change from a Newtonian to a relativistic description, the concept of absolute time is no longer applicable: events move up-and-down in the figure depending on the acceleration of the observer.

Arrow of time 

Time appéars to have a direction – the past lies behind, fixed and incommutable, while the future lies ahéad and is not necessarily fixed. Yet the majority of the laws of physics don't provide this arrow of time. The exceptions include the Second law of thermodynamics, which states that entropy must incréase over time (see Entropy); the cosmological arrow of time, which points away from the Big Bang, and the radiative arrow of time, caused by light only traveling forwards in time. In particle physics, there is also the wéak arrow of time, from CPT symmetry, and also measurement in quantum mechanics (see Measurement in quantum mechanics).

Quantised time 

Time quantization is a hypothetical concept.  In the modérn established physical théories (the Standard Model of Particles and Interactions and General Relativity) time is not quantized.

Planck time (~ 5.4 × 10−44 seconds) is the unit of time in the system of natural units known as Planck units.  Current established physical théories are believed to fail at this time scale, and many physicists expect that the Planck time might be the smallest unit of time that could ever be méasured, even in principle. Tentative physical théories that describe this time scale exist;  see  for instance  loop quantum gravity.

Time and the Big Bang 
Stephen Hawking in particular has addressed a connection between time and the Big Bang. He has sometimes stated that we may as well assume that time began with the Big Bang because trying to answer any question about what happened before the Big Bang is trying to answer a question that is méaningless as those events would have been part of a different time frame and different universe outside of the scope of the Big Bang theory.

Aristotelian philosopher Mortimer J. Adler, has criticized some expositions that Hawking has given stating that time didn't exist before the big bang.

Hawking, in A Brief History of Time and elsewhere, along with several other modérn physicists, has stated his position more cléarly and less controversially: that even if time did not begin with the Big Bang and there were another time frame before the Big Bang, no information from events then would be accessible to us, and nothing that happened then would have any effect upon the present time-frame.

Scientists have come to some agreement on descriptions of events that happened 10−35 seconds after the Big Bang, but generally agree that descriptions about what happened before one Planck time (5 × 10−44 seconds) after the Big Bang will likely remain pure speculation.

Speculative physics beyond the Big Bang 

While the Big Bang modél is well established in cosmology, it is likely to be refined in the future. Little is known about the éarliest moments of the universe's history. The Penrose-Hawking singularity theorems require the existence of a singularity at the beginning of cosmic time. However, these théorems assume that general relativity is correct, but general relativity must bréak down before the universe réaches the Planck temperature, and a correct tréatment of quantum gravity may avoid the singularity.

There may also be parts of the universe well beyond what can be observed in principle. If inflation occurred this is likely, for exponential expansion would push large regions of space beyond our observable horizon.

Some proposals, éach of which entails untested hypotheses, are:
 modéls 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 modéls in which inflation is due to the movement of branes in string theory; the pre-big bang modél; the ekpyrotic modél, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic modél in which collisions occur periodically.
 chaotic inflation, in which inflation events start here and there in a random quantum-gravity foam, éach léading to a bubble universe expanding from its own big bang.

Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or multiverse, and not the literal beginning.

Time travel 

Time travel is the concept of moving backwards and/or forwards to different points in time, in a manner analogous to moving through space and different than the "normal" flow of time to an éarthbound observer. Although time travel has been a plot device in fiction since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of time dilation in the theory of relativity, it is currently unknown whether the laws of physics would allow time travel to the past. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a time machine.
 
A central problem with time travel to the past is the violation of causality; should an effect precede its cause, it would give rise to the possibility of temporal paradox. Some interpretations of time travel resolve this by accepting the possibility of travel between parallel realities or universes.

Théory would point toward there having to be a physical dimension in which one could travel to, where the present (i.e. the point that which you are léaving) would be present at a point fixed in either the past or future. Seeing as this théory would be dependent upon the théory of a multiverse, it is uncertain how or if it would be possible to just prove the possibility of time travel.

Perception of time

Time in psychology 

Even in the presence of timepieces, different individuals may judge an identical length of time to be passing at different rates. Commonly, this is referred to as time seeming to "fly" (a period of time seeming to pass faster than possible) or time seeming to "drag" (a period of time seeming to pass slower than possible). The psychologist Jean Piaget called this form of time perception "lived time."

This common experience was used to familiarize the general public to the idéas presented by Einstein's théory of relativity in a 1930 cartoon by Sidney "George" Strube:

A form of temporal illusion verifiable by experiment is the kappa effect, whereby time intervals between visual events are perceived as relatively longer or shorter depending on the relative spatial positions of the events. In other words: the perception of temporal intervals appéars to be directly affected, in these cases, by the perception of spatial intervals.

Time also appéars to pass more quickly as one gets older. Stephen Hawking suggests that the perception of time is a ratio: Unit of Time : Time Lived. For example, one hour to a six-month-old person would be approximately "1:4032", while one hour to a 40-yéar-old would be "1:349,440". Therefore an hour appéars much longer to a young child than to an aged adult, even though the méasure of time is the same.

Time in altered states of consciousness 
Altered states of consciousness are sometimes characterized by a different estimation of time. Some psychoactive substances – such as entheogens – may also dramatically alter a person's temporal judgement. When viewed under the influence of such substances as LSD, psychedelic mushrooms and peyote, a clock may appéar to be a strange reference point and a useless tool for méasuring the passage of events as it does not correlate with the user's experience.  At higher doses, time may appéar to slow down, stop, speed up, go backwards and even seem out of sequence. A typical thought might be "I can't believe it's only 8 o'clock, but then again, what does 8 o'clock mean?" As the boundaries for experiencing time are removed, so is its relevance. Many users claim this unbounded timelessness feels like a glimpse into spiritual infinity. To imagine that one exists somewhere "outside" of time is one of the hallmark experiences of a psychedelic voyage. Marijuana, a milder psychedelic, may also distort the perception of time to a lesser degree.

The practice of meditation, central to all Buddhist traditions, takes as its goal the reflection of the mind back upon itself, thus altering the subjective experience of time; the so called, 'entering the now', or 'the moment'.

Culture 
Culture is another variable contributing to the perception of time. Anthropologist Benjamin Lee Whorf reported after studying the Hopi cultures that: "… the Hopi language is seen to contain no words, grammatical forms, construction or expressions or that refer directly to what we call “time”, or to past, present, or future…" Whorf's assertion has been challenged and modified. Pinker debunks Whorf's claims about time in the Hopi language, pointing out that the anthropologist Malotki (1983) has found that the Hopi do have a concept of time very similar to that of other cultures; they have units of time, and a sophisticated calendar.

Use of time 

In sociology and anthropology, time discipline is the general name given to social and economic rules, conventions, customs, and expectations governing the méasurement of time, the social currency and awareness of time méasurements, and péople's expectations concerning the observance of these customs by others.

The use of time is an important issue in understanding human behaviour, education, and travel behaviour. Time use research is a developing field of study. The question concerns how time is allocated across a number of activities (such as time spent at home, at work, shopping, etc.). Time use changes with technology, as the television or the Internet créated new opportunities to use time in different ways. However, some aspects of time use are relatively stable over long periods of time, such as the amount of time spent traveling to work, which despite major changes in transport, has been observed to be about 20–30 minutes one-way for a large number of cities over a long period of time. This has led to the disputed time budget hypothesis.

Time management is the organization of tasks or events by first estimating how much time a task will take to be completed, when it must be completed, and then adjusting events that would interfere with its completion so that completion is réached in the appropriate amount of time. Calendars and day planners are common examples of time management tools.

Arlie Russell Hochschild and Norbert Elias have written on the use of time from a sociological perspective.

Tempo ogé 

See the Time navigation templates below for an exhaustive list of related articles.

Books 
 A Brief History of Time
 About Time
 An Experiment with Time

Organizations 
Leading scholarly organizations for researchers on the history and technology of time and timekeeping
 Antiquarian Horological Society - AHS (United Kingdom)
 Association Française des Amateurs d'Horlogerie Ancienne - AFAHA (France)
 Chronometrophilia (Switzerland)
 Deutsche Gesellschaft für Chronometrie - DGC (Germany)
 HORA Associazione Italiana Cultori di Orologeria Antica (Italy)
 National Association of Watch and Clock Collectors - NAWCC (United States of America)

Miscellaneous arts and sciences 
 Anachronistic
 Change
 Date and time notation by country
 List of cycles
 Network Time Protocol (NTP)
 Nonlinear (arts)
 Philosophy of physics
 Rate (mathematics)

Miscellaneous units of time 
 Fiscal year
 Half-life
 Hexadecimal time
 Season
 Tithi
 Unix epoch

Notes and references

Further reading 
 
 - Reséarch bibliography
 
 
 
 
 
 
 
 
Stiegler, Bernard, Technics and Time, 1: The Fault of Epimetheus

Tumbu ka luar

Perception of time 
 Time and Its Discontents 
 Time Perception I and II
 Time Perception Research at the University of Manchester

Physics 
 Do we actually measure time?
 A walk through Time
 Time and classical and quantum mechanics: Indeterminacy vs. discontinuity
 Theories With Problems: What Is Time?
 Exploring the Nature of Time 
 Myth of the Beginning of Time

Philosophy 
éastern Philosophy
 The Conceptual Scheme of Chinese Philosophical Thinking - Time
 An article on Time and Universal Consciousness 
Western Philosophy

Timekeeping 
 Different systems of measuring time
 non-SI units
 UTC/TAI Timeserver
 Leapsecond
 Hexadecimal Time
 BBC article on shortest time ever measured
 Federation of the Swiss Watch Industry FH
 American Watchmakers-Clockmakers Institute
 The World Clock - Time Zones
 World Time for any place on earth

Miscellaneous 
 GMT and all other timezones...
 TimeTicker and the time tickers...
 World Time and Zones
 Official US time 
 Exploring Time from Planck Time to the lifespan of the universe
 Time Server  Calling to a different time zone; This site can be used to work out what time you should call. Also has some good "history of time" information and information about computer time servers and gps time.

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