This is a vast simplification. Every neuron has a different preferred baseline firing rate. Not all neurons can fire at 500 Hz. In fact, most neurons in neocortex fire at a much lower firing rate. (most neurons are in the cerebellum!)

As for the internal sense of time, there isn't a consensus as to how it is kept, but for the cognitive time scale, it seems that it is a distributed time keeping mechanism rather than some sort of central unified clock mechanism.