When a significant event occurs at a predictable time, an animal can learn to adjust its behavior in an appropriate manner. An observer can then use the animal’s behavior as a clock to guess quite accurately the time of occurrence of the significant event. What clock did the animal use to predict the event? No external clock was available, so the fact of temporal discrimination suggests that the animal has some sort of internal clock which it can read. Our general question is, what are the properties of this internal clock?