Though assisted by constructed clocks and calendars, our sense of time is actually deeply rooted in biological mechanisms we share with all other animal species (Lejeune & Wearden, 1991). Our understanding of these mechanisms has been much advanced by operant-conditioning experiments with humans and (infrahuman) animals. In these experiments, an operant response, such as key-pecking or button-pushing, is reinforced or not depending on the time elapsed since a specific stimulus (a so-calledtime marker).