The authors admire Poincaré’s idea that the convention on the nature for light propagation is the foundation of the measurement of time and point out that the conventions on the light velocity is just the convention on the symmetries of space time.The conditions on the transitivity of synchronization of clock rate are shown.With the condition,not only the question about the definition of the synchronization of rate of clocks located at different places is answered,but also find the solution to the definition of the equality of two durations is found.
