In kinematics, motion equations are derived using the time interval, Δt, between events. Before the concept of special theory of relativity came to be, everyone thought that the time interval, Δt, is the same for every observer, whether the observer is at rest or in motion. This is called absolute time.
However, according to Einstein, time is not absolute; that is, the time interval, Δt, depends upon the velocity of the observer
