Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, and into the future.It 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 a fourth dimension, along with three spatial dimensions.Time is one of the seven fundamental physical quantities in both the International System of Units (SI) 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. General relativity is the primary framework for understanding how spacetime works.Trough advances in both theoretical and experimental investigations of spacetime, it has been shown that time can be distorted and dilated, particularly at the edges 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 ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.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. In physics, time is used to define other quantities, such as velocity, so defining time in terms of such quantities would result in circularity of definitionTime in physics is operationally defined as "what a clock reads". This operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event constitutes one standard unit, such as the second, is useful in the conduct of both advanced experiments and everyday affairs of life. There are many systems for determining what time it is. 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, 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. In general, the numbers obtained from different time systems differ from one another, but with careful measurements they can be synchronized.The operational definition of time does not address the fundamental nature of time. Investigations into the relationship between space and time led physicists to define the spacetime continuum, where every event is assigned four numbers representing its time and position (the event's coordinates). Examples of events are the collision of two particles, the explosion of a supernova, or the arrival of a rocket ship. General relativity explains why the observed time of an event may be different for different observers. In general relativity, the question of what time it is now only has meaning relative to a particular observer. Distance and time are intimately related, and the time required for light to travel a specific distance is the same for all observers, as first publicly demonstrated by Michelson and Morley. Events can be separated in many directions in space, but if two events are separated by time, then one event must precede the other, and all observers will agree on this. 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. Reconciling these two theories is known as the problem of time. As of 2023, there is no generally accepted theory of quantum general relativity.