  There are two important basic questions in the measurement of time. The first
one is how to define the simultaneity of two events occurring at two different
places. The second one is how to define the equality of two durations. The
first question has been solved by Einstein, Landau and others on the convention
that the velocity of light is isotropic and it is a constant in empty space.
But no body has answered the second question until today. In this paper, on the
same convention about the velocity of light given by Poincar\'{e}, Einstein,
Landau and others, we find the solution to the definition of the equality of
two durations. Meanwhile, we also find answer to the question about the
definition of the synchronization of rate of clocks located at different
places. 04.20.Cv, 04.20.-q
