Backup system for a time of day clock in an electronic device

A backup system for a time of day clock in an electronic device that enables a microprocessor to accurately maintain the time of day clock in the event of a power interruption. The backup system can be used with any AC powered electronic device that maintains a clock. The backup system allows a capacitor to supply backup power to the microprocessor and insures the accuracy of the clock by accounting for component tolerances in the system. The backup system calibrates the time constant of an RC circuit connected to the microprocessor while AC power is still available and stores that value for later use in determining the length of a power interruption. The calibration process allows the system to use lower cost components with higher tolerance values without compromising the accuracy of the time of day clock.