Time is among the most stubbornly abstract and concretely relevant aspects of human life. Our everyday awareness is permeated with thoughts of time, and this awareness is reflected in such ordinary objects as clocks and calendars as well as in such ordinary expressions as “time flies,” “spending/ wasting/losing time,” “time heals all wounds,” and so on. Even if we had neither clocks nor maxims, there would still be the human experience of birth and death, daily and seasonal change, and the linguistic markings of a now, a before, and an after. Human beings regularly experience changes in tempo where they, or the events to which they are present, speed up or slow down and/or change from being maddeningly predictable to maddeningly unpredictable. If this were not enough, our reflections on time range all the way from the incomprehensible – eternity – to the more mundane – seconds, minutes, days, weeks. Time has been characterized as a “structural coordinate” of reality (Keen, 1970), as the “currency of life” (Hale, 1993), and as a major existential ground to human existence (Pollio, 1982). Perhaps some unknown graffiti artist put it best: “Time is nature's way of keeping everything from happening at once.”
Each discipline has its own collection of temporal units and concepts: circadian rhythms in zoology, solar and lunar cycles in astronomy, and the theories of relativity and evolution in physics and biology, respectively. In terms of human science, history often divides time into periods ranging from a few weeks – the Cuban Missile Crisis – to a decade – the Roaring Twenties – to a few hundred years – the Middle Ages.