This article describes a simple computer interface that can be used to make reliable time measurements, such as that which may be made when timing the swings of a pendulum. The interface revolves around the IBM computer printer port. It can be constructed cheaply and does not involve the modification of the internal circuits of the computer. In a sample experimental set-up, which is also described, the time measurements were found to be repeatable to three decimal places for event rates of around 14 Hz. The sample experiment described involves a form of pendulum known as the compound pendulum. The interface can be modified easily to work with other types of moving object.