The issue is, first, if there is a 'dumb' clock (and an accompanying 'correct' clock), then there would be a special reference frame in physics. What you can do

The issue is, first, if there is a 'dumb' clock (and an accompanying 'correct' clock), then there would be a special reference frame in physics. What you can do is make a convention: "I'm measuring my time in reference to the instantaneous inertial frame A of my location on Earth", and other nearby users from other inertial frames can adjust their clock rate using Lorentz transformation (I need not even mention value synchronization), such any user from A will see the clocks going at a fixed rate. Second, after adjustment, all users with different velocities will experience their clock going slower. This is the special relativity view and it creates the twin paradox: if your time is going slower and no reference frame is special, couldn't we say his time is going slower instead? Who will age faster? This isn't really a paradox in special relativity, since in SR (i.e. assuming all observers are in fixed inertial frames) the observers can't "age" to meet at a later time: their clocks will diverge, but they will get away from each other so they can never compare how they are aging -- indeed this is a symmetric position. Once you allow them to change frames so they're able to compare their own aging, some frame will have to experience acceleration, and hence break symmetry. That is, the paradox resolves in general relativity. All this is well explained on wikipedia (much better than I can do). http://en.wikipedia.org/wiki/Time_dilation http://en.wikipedia.org/wiki/Time_dilation http://en.wikipedia.org/wiki/Twin_paradox http://en.wikipedia.org/wiki/Twin_paradox