Abstract: An apparatus and method that provides an auxiliary timing source for electronic clocks, normally powered by alternating current (ac). The apparatus and method includes calibrating an inexpensive resistor-capacitor oscillator, powered by a battery. The oscillator is calibrated during normal ac operation by comparing a digital timing pulse derived from the oscillator and a digital timing pulse derived from the alternating current. When a power failure occurs, the calibrated oscillator provides an inexpensive and accurate auxiliary timing source. The apparatus and method may also include a non-volatile solid-state storage medium for storing the time of day and a set of timing instructions in the event that the battery becomes drained.