Time changes on boot up

Hello,
Whenever I boot up the computer, I get a message warning me, that the time and date are incorrect, and that this may cause problems with programs. As it does cause problems, I now have to set the time and date whenever I restart the computer. The problem is on two Macs, one is a Power Mac, and the other is a laptop. Both run on Mac OSX 10.4.8. The problem started on both computers around the same time. Neither computers are connected to the internet, although the laptop was connected a few months ago. Memory sticks which have been used on a computer, which is connected to the internet, have been used on the Macs though.
What could be causing this problem, and how do I solve it?
El

I had the same problem with my PowerMac G3, and there it was as simple as replacing a battery on the logic board. Don't know if it is that easy with your Macs, but it's worth a shot I think.

Yes as Patrick advises change your PRAM battery. For the PPC need 1/2 AA Lithium 3.6V battery available from online sellers and eBay. For the laptop check out specs for back up battery.

OK, thank you for the replies. I'm currently more familiar with PCs (windows), and Macs are rather new to me. Especially the hardware! So..pardon if I don't understand some things, and will ask comparing to the language I'm used to (in PC forums).
So, replacing the battery on the logic board means the same as replacing the battery on the motherboard? If it works like a PC, that could be a logical solution..the battery's too low to keep the time set while the system is shut down.
I have no idea what 'PRAM battery' means..
I assume, that, if I open the laptop Oops , I can just look what battery is in there, and buy the same again?
El