time and date incorrect after a shutdown???

Hello,

My sister's Macbook is set to go to sleep after 16 minutes of no activity.
She went out for half an hour and when she returned the macbook had shut down.
The date is showing as January 1st 2001 and the clock began the time at 00:00.

Everything else on the computer is fine, but she cannot correct the date and time.

When she tries restarting the computer she is given a warning that the date and time are incorrect but still cannot fix it.

Any ideas what may have happened or how to fix it?

Cheers