Real-time systems could be modeled using AOP based on UML. Timing requirements could be separated from the system according the separation of concerns techniques, expressed as a time-aspect independence of the system, and designed occurrence. So a timing model could be created to describe the time of the system. Finally the timing model could be woven into the system to compose a real-time system based on the AOP technology only when needed for a particular application. Also UML had been extended to express AOP and the time model. The real-time systems could be modeled from the static structure, dynamic behaviors and weaving of the time-aspect, and an elevator case had been given as an example.