Research on the mental representation of time ('subjective time') has provided broad insights into the nature of time perception and temporal processing. As the field comprises different scientific disciplines, such as psychology, philosophy, and neuroscience, studies differ with regard to the basic terms and concepts used. For this reason, research on subjective time lacks a coherent conceptual system. We argue that research in the field of subjective time should aim at establishing such a system, i.e., a more standardized terminology, in order to strengthen its theoretical basis and to support an efficient communication of results. Based on key empirical findings and concepts that are commonly (but inconsistently) used in the literature, we argue for a conceptual framework for the study of subjective time that differentiates between three types of mental representations of time: basic temporal processing, time perception in terms of passage, and time perception in terms of duration.