I'd say it's the little battery that keeps the BIOS settings in check may have gone flat or the hard disk is beginning to show signs of going permanently kaput. Both have a finite life. Not sure how tech savvy you are but if you can find out the time during boot up and it is wrong by a number of years then it's probably the little battery.
If it is the battery then where they put that thing and how to change it needs some googling time.