The Unified Modeling language provides a robust set of tools for modeling software systems. However, these tools do not directly address the requirements of real-time systems. Many real-time systems require the specification of data that has strict timing constraints. This paper presents a UML package for specifying real-time objects called RT-Object. The constructs in the package are based on the objects of the RTSORAC (Real-Time Semantic Objects Relationships And Constraints) model. The RT-Object package has been used to design real-time objects in a Real-Time Multi-User Virtual Environment in which widely distributed users collaborate in time-critical planning and decision making. A version of this paper appears in Computer Standards and Interfaces 22 (2000) 307-321