The theory is concerned with time and distance measurements occurring across differing reference frames. There is no absolute time.
Time is relative to the frame of reference in which a time interval is observed.
It has been verified that clocks in a moving object run slower than a reference clock that is at rest. This effect is called time dilation. The formula below derives the time interval between events in a moving frame of reference as measured by a stationary observer, given the relative speed of the moving reference frame and the same time interval as measured from within the moving reference frame. Let T be the time interval measured from a stationary frame of reference. Let T0 be the time interval measured within a moving frame of reference.