Take a rate of oscillation of a wrist watch crystal at rest and compare with rate of moving one. Due to symmetry (invariance) of all natural laws versus state of inertial motion you find that speeding clocks SHOULD slow down compared to stationary clocks (and vice versa - moving observer finds that STATONARY clock should slow down). That ratio (of two numbers corresponding to number of cycles of oscillations of each crystal as seen by an observer) is what we call "time".

So, in essense, time is not a physical entity like energy or mass, it is just a mathematical entity (well, at close look energy and mass also are purely mathematical entities too).