A formal design notation is presented whose underlying computational model is object-based. The object structure of the model is based on the practical, industry-strength Object-Oriented structured development technique HRT-HOOD. The computational model has been specifically chosen because it leads to designs which can be analyzed for their schedulability in a distributed hard real-time execution environment. It is a wide-spectrum language supporting abstract description statements in Interval Temporal Logic (ITL) for the description of the timing, functional, and communication behavior of the proposed real-time system, and concrete Temporal Agent Model (TAM) statements with objects which can be directly executed. The semantics of these concrete statements is defined denotationally in specification-oriented style using ITL. A system specified at a high level of abstraction can be systematically transformed into an executable program by the use of sound ITL refinement rules.