Hi,
Most likely the CMOS battery on the PC or laptop you have is low or dead, and is therefore not capable of keeping the date/time stored in the BIOS. If you wouldn't mind telling us the Make/Model of your PC or laptop, we can assist you with how to change the battery. It's a $6 item at Radio Shack or most pharmacy's such as Walgreens or CVS. It can take quite a bit more work to get to in order to change in laptops, so you may wish to take it to a licensed Computer Tech such as Geek Squad and pay them to do for you.
Here's a little video on how to do on an older laptop if you decide to change the Battery yourself or you have done it before on a PC:
how to change a cmos batter in a pc - Bing video
And if you have a Desktop...