Time dialation

In the theory of relativity, time dilation is a difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from a gravitational mass or masses. An accurate clock at rest with respect to one observer may be measured to tick at a different rate when compared to a second observer's own equally accurate clocks. This effect arises neither from technical aspects of the clocks nor from the fact that signals need time to propagate, but from the nature of spacetime itself.

Einstein's theory of relativity

The theory of relativity was postulated by Albert Einstein in 1905 , it had two postulates
1: that the laws of physics are thesame in all inertial reference frames..I.e inertial frames are where newtons laws of motion(classical mechanics) are applicable
2: that the speed of light in vacuum is thesame for all observers . This means that the speed of light is invariant no matter the time, place where it is been checked .