Time

Time is the continuous progression of existence that occurs in an apparently irreversible succession from the past, through the present, and into the future. Time dictates all forms of action, age, and causality, being a component quantity of various measurements used to sequence events, to compare the duration of events, and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.
Time is primarily measured in linear spans or periods, ordered from shortest to longest. Practical, human-scale measurements of time are performed using clocks and calendars, reflecting a 24-hour day collected into a 365-day year linked to the astronomical motion of the Earth. Scientific measurements of time instead vary from Planck time at the shortest to billions of years at the longest. Measurable time is believed to have effectively begun with the Big Bang 13.8 billion years ago, encompassed by the chronology of the universe. Modern physics understands time to be inextricable from space within the concept of spacetime described by general relativity. Time can therefore be dilated by velocity and matter to pass faster or slower for an external observer, though this is considered negligible outside of extreme conditions, namely relativistic speeds or the gravitational pulls of black holes.
Throughout history, time has been an important subject of study in religion, philosophy, and science. Temporal measurement has occupied scientists and technologists, and has been a prime motivation in navigation and astronomy. Time is also of significant social importance, having economic value as well as personal value, due to an awareness of the limited time in each day and in human life spans.

Definition

The concept of time can be complex. Multiple notions exist, and defining time 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. Traditional definitions of time involved the observation of periodic motion such as the apparent motion of the sun across the sky, the phases of the moon, and the passage of a free-swinging pendulum. More modern systems include the Global Positioning System, other satellite systems, Coordinated Universal Time and mean solar time. Although these systems differ from one another, with careful measurements they can be synchronized.
In physics, time is a fundamental concept to define other quantities, such as velocity. To avoid a circular definition, time in physics is operationally defined as "what a clock reads", specifically a count of repeating events such as the SI second. Although this aids in practical measurements, it does not address the essence of time. Physicists developed the concept of the spacetime continuum, where events are assigned four coordinates: three for space and one for time. Events like particle collisions, supernovas, or rocket launches have coordinates that may vary for different observers, making concepts like "now" and "here" relative. In general relativity, these coordinates do not directly correspond to the causal structure of events. Instead, the spacetime interval is calculated and classified as either space-like or time-like, depending on whether an observer exists that would say the events are separated by space or by time. Since the time required for light to travel a specific distance is the same for all observers—a fact first publicly demonstrated by the Michelson–Morley experiment—all observers will consistently agree on this definition of time as a causal relation.
General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. In quantum mechanics, time is treated as a universal and absolute parameter, differing from general relativity's notion of independent clocks. The problem of time consists of reconciling these two theories. As of 2025, there is no generally accepted theory of quantum general relativity.

Measurement

Methods of temporal measurement, or chronometry, generally take two forms. The first is a calendar, a mathematical tool for organising intervals of time on Earth, consulted for periods longer than a day. The second is a clock, a physical mechanism that indicates the passage of time, consulted for periods less than a day. The combined measurement marks a specific moment in time from a reference point, or epoch.
Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities. The SI base unit of time is the second, which is defined by measuring the electronic transition frequency of caesium atoms.

History of the calendar

Artifacts from the Paleolithic suggest that the moon was being used to reckon time by at least 6,000 years ago. Lunar calendars were among the first to appear, with years of either 12 or 13 lunar months. Without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year and a year of just twelve lunar months. The numbers twelve and thirteen came to feature prominently in many cultures, at least partly due to this relationship of months to years.
Other early forms of calendars originated in Mesoamerica, particularly in ancient Mayan civilization, in which they developed the Maya calendar, consisting of multiple interrelated calendars. These calendars were religiously and astronomically based; the Haab' calendar has 18 months in a year and 20 days in a month, plus five epagomenal days at the end of the year. In conjunction, the Maya also used a 260-day sacred calendar called the Tzolk'in.
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 year. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but it is now by far the most commonly used calendar around the world.
During the French Revolution, a new clock and calendar were invented as part of the dechristianization of France and to create a more rational system in order to replace the Gregorian calendar. The French Republican Calendar's days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the base 12 system used in many other devices by many cultures. The system was abolished in 1806.

History of other devices

A large variety of devices have been invented to measure time. The study of these devices is called horology. They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means.
A sundial is any device that uses the direction of sunlight to cast shadows from a gnomon onto a set of markings calibrated to indicate the local time, usually to the hour. The idea to separate the day into smaller parts is credited to Egyptians because of their sundials, which operated on a duodecimal system. The importance of the number 12 is due to the number of lunar cycles in a year and the number of stars used to count the passage of night. Obelisks made as a gnomon were built as early as. An Egyptian device that dates to, similar in shape to a bent T-square, also measured the passage of time from the shadow cast by its crossbar on a nonlinear rule. The T was oriented eastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction.
Alarm clocks reportedly first appeared in ancient Greece with a water clock made by Plato that would set off a whistle. The hydraulic alarm worked by gradually filling a series of vessels with water. After some time, the water emptied out of a siphon. Inventor Ctesibius revised Plato's design; the water clock uses a float as the power drive system and uses a sundial to correct the water flow rate.
In medieval philosophical writings, the atom was a unit of time referred to as the smallest possible division of time. The earliest known occurrence in English is in Byrhtferth's Enchiridion of 1010–1012, where it was defined as 1/564 of a momentum, and thus equal to 15/94 of a second. It was used in the computus, the process of calculating the date of Easter. The most precise timekeeping device of the ancient world was the water clock, or clepsydra, one of which was found in the tomb of Egyptian pharaoh Amenhotep I. They could be used to measure the hours even at night but required manual upkeep to replenish the flow of water. The ancient Greeks and the people from Chaldea 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. In the 11th century, Chinese inventors and engineers invented the first mechanical clocks driven by an escapement mechanism.
Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Water clocks, and, later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. The passage of the hours at sea can also be marked by bell. The hours were marked by bells in abbeys as well as at sea. Richard of Wallingford, abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330. The hourglass uses the flow of sand to measure the flow of time. They were also used in navigation. Ferdinand Magellan used 18 glasses on each ship for his circumnavigation of the globe. The English word clock probably comes from the Middle Dutch word klocke which, in turn, derives from the medieval Latin word clocca, which ultimately derives from Celtic and is cognate with French, Latin, and German words that mean bell.
Great advances in accurate time-keeping were made by Galileo Galilei and especially Christiaan Huygens with the invention of pendulum-driven clocks along with the invention of the minute hand by Jost Burgi. There is also a clock that was designed to keep time for 10,000 years called the Clock of the Long Now. Alarm clock devices were later mechanized. Levi Hutchins alarm clock has been credited as the first American alarm clock, though it can only ring at 4 a.m. Antoine Redier was also credited as the first person to patent an adjustable mechanical alarm clock in 1847. Digital forms of alarm clocks became more accessible through digitization and integration with other technologies, such as smartphones.
File:ChipScaleClock2 HR.jpg|thumb|Chip-scale atomic clocks, such as this one unveiled in 2004, are expected to greatly improve GPS location.
The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years, and are used to calibrate other clocks and timekeeping instruments. Atomic clocks use the frequency of electronic transitions in certain atoms to measure the second. One of the atoms used is caesium; most modern atomic clocks probe caesium with microwaves to determine the frequency of these electron vibrations. Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms. SI defines the second as 9,192,631,770 cycles of the radiation that corresponds to the transition between two electron spin energy levels of the ground state of the ¹³³Cs atom. A portable timekeeper that meets certain precision standards is called a chronometer. Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation, a precision first achieved by John Harrison. More recently, the term has also been applied to the chronometer watch, a watch that meets precision standards set by the Swiss agency COSC.
In modern times, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe., the smallest time interval uncertainty in direct measurements is on the order of 12 attoseconds, about 3.7 × 10²⁶ Planck times. The time measured was the delay caused by out-of-sync electron waves' interference patterns.