Guaranteeing timing properties is an important issue as we develop safety-critical real-time systems such as cardiac pacemakers. We present a safety assured development approach of real-time software using a pacemaker as our case study. Following the model-driven development techniques, measurement-based timing analysis is used to guarantee timing properties in implementation as well as in the formal model. Formal specification with timed automata is checked with respect to timing properties by model checking technique and is transformed into implementation systematically. When timing properties may be violated in the implementation due to timing delay, it is suggested to measure the time deviation and reflect it to the code explicitly by modifying guards. The model is altered according to the modifications in the code. These changes of the code and the model are considered safe if all the properties are still satisfied by the modified model in re-performed model checking. We demonstrate how the suggested approach can be applied to single-threaded and multi-threaded versions of implementation. This approach can provide developers with a useful time-guaranteeing technique applicable to several code generation schemes without imposing many restrictions.