In recent years there has been a growing interest in the explicit introduction of time modelling in a conceptual schema. This has come about as a result of a realization that the development of large information systems is becoming increasingly more difficult as user requirements become broader and more sophisticated. Arguably the most critical activity in the development of a large data-intensive information system is that of requirements capture and specification. The effectiveness of such a specification depends largely on the ability of the chosen conceptual model to represent the problem domain in such a way so as to permit natural and rigorous descriptions within a methodological framework. The explicit representation of time in a conceptual model plays a major role in achieving this effectiveness. This paper examines the ontology and properties of time in the context of information systems and conceptual modelling. In particular, a critical set of ontological assumptions about time are examined which in turn give rise to the way different paradigms deal with time modelling. The paper describes and critically evaluates nine contemporary approaches to the specification and use of time in information systems.