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Do you mean the clock that tells the time? (rather than the system speed)
If so, is it just running continually slowly, so that the time gradually falls behind what it should be?
It is normal for the RTCs (Real Time Clocks) on PCs to be inaccurate. If this is what is troubling you, set the PC to update time daily from an internet timeserver. In Windows, open the time/date box and click the Internet Time tab. Use a server like time.windows.com, or search google for time servers near you.
Manish, you are absolutely right. Clocks (RTCs) in desktop *and* notebooks have a small coin cell to keep the time when there is no power. You can confirm this by removing the main battery in a working laptop; it would still maintain correct time.
What usually happens is when this battery falls under a certain level, it cannot keep the RTC running by itself, but it would retain the last time as when power was switched off. Therefore, the next time you switch on the laptop, it looks like it has 'lost' time.
The solution, of course, is to replace it. Unfortunately, on a notebook it could be as simple as opening one of the bay covers at the bottom, or having to open the entire thing.
If the battery is not very accessible, go with Rick's solution of synchronizing your laptop with a time server, say every hour or so.