Mac doesn't update time after waking up

[Phelpsx]

I don't know if this is the right place to post my question.
But something has been driving me crazy for the last weeks.

Let's say I close my macbook at 11AM. And then I go home from my class.
And I get home at 11:30AM, and then I open my macbook again.

It doesn't update the time to 11:30AM.

I missed two classes already because of that.
I'm using ML 10.8.2 in a MacBook Pro Late 2011.

I tried both checking and unchecking the "automatically adjust time".

I have no idea what to do.

Thanks!

 

[Phelpsx]

Thanks for all the answers.

 

[shoulin333]

sounds like bad cmos battery.

 

[Phelpsx]

I don't think that can be the problem.
I have my macbook for less than a year.

 

[nzreg]

Do you have istat menu installed? this stopped my clock from updating the time.