This tag is for questions regarding the 'time dilation' which is the lengthening of the time interval between two events for an observer in an inertial frame that is moving with respect to the rest frame of the events (in which the events occur at the same location).

Time dilation, in the theory of special relativity is the “slowing down” of a clock as determined by an observer who is in relative motion with respect to that clock.

The equation for calculating time dilation is as follows: $$t = t_0/(1-v^2/c^2)^{1/2}$$where: $t =$ time observed in the other reference frame
$t_0 =$ time in observers own frame of reference (rest time)
$v =$ the speed of the moving object
$c =$ the speed of light in a vacuum