You can create a 'clock' using a discrete integrator. For example if you can set the time-step of the integrator equal to model sample time, and feed it a constant input of 1 it will act like a clock.
The initial condition of the integrator can be used to control the starting value of the 'clock', and the reset condition can be used to control when the timer starts (or resets). Therefore you can feed the output from your first 'clock' into a compare block in order to generate the necessary reset signal (to start your second 'clock').
