A temporal situation can be described at different levels of abstraction depending on the accuracy required or the available knowledge. Time granularity can be defined as the resolution power of the temporal qualification of a statement. Providing a formalism with the concept of time granularity makes it possible to model time information with respect to differently grained temporal domains. This does not merely mean that one can use different time units, e.g., months and days, to represent time quantities in a unique flat temporal model, but it involves more difficult semantic issues related to the problem of assigning a proper meaning to the association of statements with the different temporal domains of a layered temporal model and of switching from one domain to a coarser/finer one. Such an ability of providing and relating temporal representations at different "grain levels" of the same reality is both an active research theme and a major requirement for many applications (e.g., integration of layered specifications and agent communication). After a presentation of the general requirements of a multi-granular temporal formalism, we discuss the various issues and approaches to time granularity proposed in the literature. We focus our attention on the main existing formalisms for representing and reasoning about quantitative and qualitative time granularity: the set-theoretic framework developed by Bettini et al. [Bettini et al., 2000] and the logical approach systematically investigated by Montanari et al. [Montanari, 1996; Franceschet, 2002] for quantitative time granularity, and Euzenat's relational algebra granularity conversion operators [Euzenat, 2001] for qualitative time granularity. We present in detail the achieved results, we outline the open issues, and we point out the links that connect the different approaches. In the last part of the chapter, we describe some applications exploiting time granularity, and we briefly discuss related work in the areas of formal methods, temporal databases, and data mining.
