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.