CMOS reset on startup

Hello everyone. For some time now, every time I turn on my HP netbook, a message appears for a few seconds: "Error 502 CMOS reset, press enter to reboot." I press enter, and once the desktop is displayed, the time is set to 00:00 on January 1, 2010. I haven't (for now) lost my files...

Could someone please enlighten me about this problem? What should I do?

Thank you in advance.