Abstract: The issues about ＂the simultaneity of two events occurring at two different places＂ and ＂the equality of two durations＂ given by Poincaré and others are introduced. The programme defining simultaneity of clocks located at different places suggested by Einstein, and developed by Landau and others, is presented. We admire Poincaré＇s idea that the convention on the nature for light propagation is the foundation of the measurement of time, and we point out thai the convention on the light velocity is just the convention on the symmetries of space time. The condition on the transitivity of synchronization of clock rate is shown by us on the convention about the light velocity. With the condition, we not only answer the question about the definition of the synchronization of rate of clocks located at different places, but also find the solution to the definition of the equality of two durations. Poincaré＇ s original guess is confirmed. The theoretical foundation on the measurement of time is simplified.

Key words: simultaneity, synchronization of clock rate, duration, measurement of time, velocity of light