Real-time constraints are required typically in embedded systems. They can be regarded as crosscutting concerns and thus make real-time programs bulky. The goal of this work is to propose a description method of real-time constraints and their modularization technique using the idea of context-oriented programming. We also design a programming language ProcneJ for real-time systems based on the method. It is natural to think that a real-time (embedded) program depends not only on its changing runtime environment, but also on the passage of time. Thus we regard time points and time intervals as contexts and their changes as events to apply the idea of event-based context-oriented programming (as EventCJ) to real-time programs. In our language ProcneJ, separating the descriptions of event-based state transitions and class definitions enables modular development of real-time programs. In addition, by generating timed automata from the state transition parts, we can verify the consistency of real-time constraints using UPPAAL model checker. In the presentation, we will present the basic idea of our method and will show how it is beneficial for describing real-time systems through examples.