Measuring clock jitter

The invention provides a technique of correcting time data based on a clock signal affected by jitter. The error due to jitter in a time measurement of an event in the clock signal (17) is determined at the time of the event or as an average over a number of events. A measurement is made of a time dependent reference variable associated with a long-time constant device (19), such as a capacitor, which is relatively immune to localised jitter. The measurement may be a reading of the voltage across a charging capacitor. The measured value is compared to an expected value and the time error is based on the result. The expected value may be known from look-up tables (18) in the memory (6) of the device (1) comprising the capacitor (19) or calculated from known charging rates of the capacitor. The error due to jitter of a time measurement is approximately linearly proportional to the difference in voltage between the measured and the expected values of the capacitor. This technique assists in removing any contribution of error to a Time of Flight measurement between two nodes in a network, caused by variations in localised jitter within the nodes.