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There is a sensor on your car that monitors drainage from the battery overnight (there should be next to none). If there is drainage on the battery, or the battery is very low, then this sensor will reset all of the computer modules in the car (including the clock) in an attempt to stop the drain / save the battery.
So, if the clock is resetting, this would tend to indicate that there is an overnight draw on the car – either that or the battery is weak or the battery sensor is faulty (uncommon for the battery sensor to be faulty, but possible).
So, at this time, you can consider either replacing the battery (requires the car be reprogrammed after replacement so that the new battery is charged and monitored properly) or have further testing carried out to determine if there is a drain on the battery, and if so, what is the cause.
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