The current model of time in multimedia frameworks poses particular problems when designing multimedia systems with timebased interaction. We propose to expand and extend an existing distinction between semantic time and real time from music and film theory to multimedia systems design. The semantic time concept also forms the foundation of a new software framework for multimedia systems that we are building, that, unlike most existing frameworks, includes mechanisms for time-based effects such as time-stretching. Categories and Subject Descriptors: D.2.11: Software Architectures – Patterns; D.2.2: Design Tools and Techniques – Software Libraries, User Interfaces General Terms: Design, Theory