We present a calculus which can transfer specifications to objects for the development of real-time systems. The object model is based on a practical OO development technique--HRT-HOOD. A real-time logic is specified by extending a sound formal method for real-time systems-TAM, to formalise the object model. With integration of HRT-HOOD and TAM, the advantages of object-oriented structured methods with the stepwise refinement techniques are combined. The result is illustrated on a case study.