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Many electronic devices (e.g., VCR) don't turn off when the power switch is cycled. They are always powered up, but are put into standbye mode when you turn them 'off'. Therefore, the clock is always running until the power is actually removed from the device. If such a device is disconnected from the mains or if the power fails, it may lose the time of day information or, if it uses a clock with a battery backup, the battery will keep the clock running until the battery is discharged. Power is conserved by turning the visual display off. Such a system may use a small conventional battery, a small rechargable battery, or a super-capacitor to power the clock until the power is restored. Some devices use two clocks: one that has a battery or super-capacitor to retain the time and one that shows the current time. When you power up such a device, it reads the time from the 'backup' clock and loads these values into the 'visible' clock. If you change the time on such a system, both clocks are updated.
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