The problem you describe is indicative of a dead/dying/failing backup battery in a Mac. This does not normally happen to a PB. If the PB battery is operating correctly, does not go flat overnight, then this problem should not occur.
The PB battery supplies power to the PRAM, time/date/other bits, and unless this battery is dead, they should not revert to 1970.

There should also be no reason why you should have to restart the machine to get the date correct. If you have the Date/Time preference pane set to get the time automatically, then connecting to the internet will fix the incorrect time.
Now, there may be something else that you have not mentioned, like not being able to connect to the internet without rebooting. Usually via wireless.

Here is a suggestion. Keep the AC power adapter plugged into the machine overnight and see it looses the the date/time again. With constant power from the adapter, it should not.
How long does the battery last for when the AC is unplugged?

P