While you're on the topic, don't ever turn your tablet off.

There's absolutely no reason to turn the tablet off unless you need to restart it. Turning the tablet off doesn't save battery life because the process of switching it on is equivalent to several days on standby. If you need to stop it activating while it's in a bag or pocket, lock the touchscreen and keys by pressing the power button briefly and selecting that option (or on the N810 just flick the lock switch).

I've just tried my N800 with the LCARS theme and the date and time don't reset when I switch it off and on, so I don't know what the problem might be with your tablet. You're not removing the battery are you?