This paper discusses fundamentals of hybrid system modeling. Emphasis is put on compositionality and the use of multiform time. Compositionality refers to the ability of freely composing hybrid systems. Since hybrid systems are considered, different time indexes occur, and modularity calls for considering "time" as local to each module, this is what is called multiform time. The proposed framework is behavioral, hence a way is provided to automatically synthesize proper scheduling constraints for the joint simulation of discrete-time, event-based, and continuous-time components. Finally, the relations of this model with the more traditional state-based point of view are discussed.