Yeah I get the whole private key and encryption principle. Basic cryptology. I never said anything about syncing those over the network.
So the client generates new private keys every once in a while, and that's the reason the backup can't be too old. Thanks for that info, perfectly clear to me now!
Thanks again!
The client only generates new private keys when you create a new receiving address. If you don't do that, the backup can be as old as you want, I believe.
it also generates addresses when you send a transaction. (for the change)