Does the clock run normally when the computer powered on? If yes, the clock chip should be fine.

Does the PRAM keep settings properly? If not, there might be something wrong with the battery connection.

If the PRAM setting are kept, the battery connection should be okay. Still there might remain a damaged wire to supply the clock chip.

Check also for unusual software or software settings. For example there could be a control panel called "Clock Adjust" which might cause unexpected results if not set properly. Could someone have put a program to reset the clock into the startup items folder? Consider to shut down and boot with extensions disabled.