You have to compare two clocks. Another example of time dilation is this: take two really precise clocks, carry one up a mountain and back, and compare it to t

You have to compare two clocks. Another example of time dilation is this: take two really precise clocks, carry one up a mountain and back, and compare it to the clock that stayed at sea level. The one at sea level will have run slower and be behind compared to the other clock because it spend some time in a place where gravity was less. Here is a guy that did exactly that in 2005 on a tour using his van. He even took his kids: http://www.leapsecond.com/great2005/tour/ http://www.leapsecond.com/great2005/tour/