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If I had to take a guess, I suspect the clock is just an internal circuit within the radio unit and I suspect the only way to fix that is to have the radio serviced or replaced ... but I could be wrong. Good luck, hope you get to the bottom of it quickly and painlessly.
BTW - I assume you are referring only to the clock on the radio. Have you checked the Trip elapsed time function within the DIC ? I suggest resetting both at the same time and then compare together to see if perhaps they are in sync. If so, then there could be a bigger problem more integral to the cars computer itself.