In order to study the formal nature of real-time systems, various mathematical models that use temporal constraints have been proposed. This paper examines a collection of properties that such a model should possess. Specifically, it should model the logic’s expressiveness, the logic’s order, the presence of a metric for time, the type of temporal operators, the fundamental time entity, and the structure of time. Sixteen temporal logics are discussed in terms of these properties, and most of them are found not to be fully satisfactory for the specification of real-time systems. The authors provide examples in addition to a general discussion of temporal logics.