For thousands of years before the invention of reliable clocks, humans measured their days by the motion of the sun. Astronomically, one day was the length of time it took for the sun to return to the same position in the sky. With the advent of precise mechanical chronometers such as Harrison's timekeepers (Sobel and Andrewes 1998), which ran at a predictably regular rate, it was soon discovered that the length of a solar day varies. The difference between apparent solar time and mean solar time, which varies throughout the year, is known as the "equation of time." In this article, the author demonstrates how students make use of cell phones and graphing calculators to model the differences between solar and clock time. (Contains 4 figures and a bibliography.)