General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. 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. In general relativity, the question of what time it is now only has meaning relative to a particular observer. However, the numerical values are different for different observers. Every event can be 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. The physical nature of time is addressed by general relativity with respect to events in spacetime. Time in physics is operationally defined as 'what a clock reads'. 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. 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.

Time is often referred to as a fourth dimension, along with three spatial dimensions. 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 the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future.

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