Some real-time systems are designed to deliver services to objects that are controlled by external sources. Their services must be delivered on a timely basis, and the system fails when some services are delivered too late. Such a system may fail if the timing requirements which it is designed to meet are erroneous. It may under-utilize resources and, consequently, be costly or unreliable if the requirements are too stringent. In general, the timing requirements of the system may change when the states of the objects monitored by the system change. Hence, one must identify how changes in object states call for changes in system requirements and how these changes should be incorporated in the design and implementation of the system. This paper first describes a methodology to determine timing requirements and to take into account of requirement changes at runtime. The method is based on several timing requirement determination schemes. Simulation data show that these schemes are effective for applications such as mobile IP hand-offs. The paper then discusses how to incorporate this methodology in the design of such systems and in the development process.