This paper describes a longitude system for Saturn which is locked to the period of Saturn kilometric radiation. Because the apparent radio emission period varies with time, the period used in the system is allowed to vary. The resulting system results in the 'diurnal' peak of the radio emission occurring when the subsolar longitude is 100¿, as was the case during the Voyager epoch. The variable period used in this system is shown to be statistically the same as periodicities recently reported for residuals in Saturn's magnetic field. It is expected that this longitude system will be more useful for organizing magnetospheric phenomena and even spoke creation in the rings than the existing longitude system based on the fixed period determined from Voyager observations which is fully 1% shorter than the currently-measured period.