Time keeps resetting

Hello,
Whenever I turn on my computer, I get a message saying the configuration is "bad" and that the date and time aren't set. I've changed and saved the correct (give or take a minute) date and time on the setup menu but whenever I start the computer the same message appears and the time has gone back to the default settings....please help. I don't restart the computer or anything, I will have a regular session on the computer and shut it down properly before switching it off, but I can't understand why I have to keep going through this annoying process whenever I turn the computer on.