We propose a first-order model as a possible formal basis for normative agent systems (NAS). The model allows us to describe the execution of actions in time and the use of dynamic norms. We also present its operational aspect which is based on the reduction of the deontic constraints to temporal ones. In order to show the applicability of the model we briefly describe a logic programming implementation and we present two types of applications: the detection of the violation cases and the prototype of a scheduler with deontic and temporal constraints.