The specification and monitoring of conditional obligations and prohibitions with starting points and deadlines is a crucial aspect in the design of open interaction systems. In this paper we regard such obligations and prohibitions as cases of social commitment, and propose to model them in OWL, the logical language recommended by the W3C for Semantic Web applications. In particular we propose an application-independent ontology of the notions of social commitment, temporal proposition, event, agent, role and norms that can be used in the specification of any open interaction system. We then delineate a hybrid solution that uses the ontology, SWRL rules, and a Java program to dynamically monitor or simulate the temporal evolution of social commitments, due to the elapsing of time and to the actions performed by the agents interacting within the system.