In the previous chapter we introduced structures to describe events, synthesis techniques, and control functions. The system still lacks structures to organize these elements into a coherent composition. Furthermore, the time, as it is represented in the formal model, is linear. However, in certain cases we want to relate the time in the composition to the ``absolute time'' in a non-trivial fashion. In this chapter we propose a set of classes to describe the relations and behavior in the time organization of a music piece. Our propositions are based on the works discussed in chapter 3. Many of the problems cited in that chapter find an elegant solution by adopting a rich time model. The first part of this chapter will therefore only consider time. This discourse will be applicable to any time-based media. In the second half of the chapter we will focus on the particular case of sound synthesis. We start with a description of activities.