Time is the interval between two events Time is the interval between two events, usually expressed in seconds, minutes, hours, etc. The concept of time is a very important concept in physics and astronomy. Together with space, it forms the basic structure of the universe. In the theory of relativity, time is considered relative, and the passage of time depends on the motion of the observer and the strength of the gravitational field, which also leads to the famous time expansion effect. Time was also widely used in time travel, causality, and other fields.