Hello fabtique!
The reason your iBook date and time are reseting is due to a battery located on the logic board of your iBook. Just like a watch battery it has died because of how old your iBook is. Noting that the most recent iBook before it was discontinued was made in 2005, which is now 5 years ago, The earliest being made in 2003, 7 years ago. Alternately I think (don't know) it might have something to with the computer's PRAM which I think is responsible for things like Time and date as well as sleep (IDK)
So all in all, The solution lies in replacing the battery which has obviously died on your logic board here are two links that will tell you how to open you iBook and another for purchasing a new battery. Alternatively you could take it into a Apple Store and have it repaired (It's not obsolete yet) Deal with the issue or fix it. PS You don't need to buy the battery online they can be found at local shops (Watchmakers)
Here are the links:
http://www.ifixit.com/Guide/Browse/iBook
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http://support.apple.com/kb/ht1379
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http://www.mac-batteries.com/ (I could not load the page so I don't know if it's relevant)
Ed