"any ray of light moves in an 'stationary' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body." thus, the distance a ray of light travels during an event serves as the best clock to measure the time of the event. for example, let there be given two rays of light, r1 and r2. the distance either one of these rays of light travels can serve as a clock, and the distance the other travels can serve as an event. let us arbitrarily choose the distance r1 travels to serve as a clock and the distance r2 travels as an event. we can now define the second as the distance r1 travels (299,792,458 meters) as r2 travels simultaneously 299,792,458 meters. thus, the time of an event is the distance a ray of light travels during the event. thus, 1 second is equivalent to 299,792,458 meters.