In a time-triggered system, activities like task releasing, operational mode switches, sensor readings and actuations are all initiated at predetermined time instants. This paper proposes an extension of the TDL (Timing Definition Language) time- triggered compositional framework, and presents, based on the widely-applied methods, a condition for its schedulability. The schedulability condition developed accounts for multiple concurrently executing modules, multiple operational modes and mode switches. This way the system schedulability can be guaranteed in any execution condition.