Time is an important but complex area of learning for students in the primary years. This study sought to develop a tool which would provide a clear picture of students' understanding of what constitutes time and how time is related to clock and calendar use. Four major components of time, Awareness of time, Succession, Duration, and Measurement of time, with related key ideas, formed the basis for a Framework for the Learning and Teaching of Time. This Framework underpinned the development of a 69-item, one-to-one task-based interview to assess students' understanding of time. Data from interviews of Year 3 and 4 students gave a clear picture of how they experienced the mathematics of time. The range of scores across all assessed areas of the Framework revealed a considerable spread of students' understandings of time concepts. This paper focuses on the development of the Framework and interview, the interview data, and the general benefits of the use of the task-based assessment interview in assessing the mathematical understanding and thinking of children regarding time.