The battery was good for about a decade ... device with some tidily soldered Kynar wires to bring in the power. A CR1225 cell was used rather than the ubiquitous CR2032, as space was at a premium ...

The CR1220, CR1225 or LIR1220 (recommended) backup battery keeps the clock running even without an external power supply. The built-in 32.768 kHz crystal ensures a reasonable accuracy of the clock.