This paper presents a temporal model for the coverage achieved by software testing. The proposed model, which is applicable at any level of the testing hierarchy, can determine the value of test coverage at any given time, as well as predicting future values. The model is comprised of two main components: coverage functions, and the coverage matrix. The coverage functions represent the coverage of a single entity as a function of time and reflect the test environment through their stochastic parameters. The coverage matrix utilizes the coverage functions to depict the coverage attained for each entity by each test within the test suite. A normalized sum of the elements of the coverage matrix is used to represent the overall coverage achieved by the test suite, as a function of time. The application of the model to multi-phase testing is illustrated. In the application section, test coverage values from Y2K compliance testing are used to verify model predictions.