Heidegger claims that the traditional theory of time is based on a failure to understand the nature of time. In this chapter I develop and evaluate this thesis. As an account of temporal experience, Heidegger's conception of temporality is avowedly revisionist. It depends on rejecting the intuitive claim that time is fundamentally now-centered. Heidegger offers an interesting rationale for the thesis that what exists now should not be taken to be as fundamental to experience as we tend to take it to be. This amounts to a critique of the traditional manner in which time is interpreted in terms of tense. The notion of tense is generally thought to involve a now relative to which a present, and then a past and future may be defined. Heidegger argues that this notion of time as a succession of nows is highly derivative. It can only be adequately understood in terms of a notion of tense that is based on a quite different notion of presence. Heidegger's work also suggests a rather subtle critique of the most obvious alternative to the conception of time based on tense. This alternative theory of time based on physical theory construes time as the tenseless existence of events at certain clock-times.

In articulating Heidegger's conception of time I shall proceed as follows: first I discuss what he refers to as the “vulgar” notion of time. This is the notion of time that has dominated the philosophical tradition and the natural sciences. It is based on the assumption that time, regardless of whether it is identified with tense or not, is something that is essentially measurable by clocks.