Does it happen in front of your eyes or when left unattended . . . or are you experiencing random reboots and after reboot the date/time is off?
If its happening when the tablet isn't in use and you notice it on first wake take a look at the recent tasks list to see if anything is running. If the task list is clean you may have experienced a reboot.
Point of the questions is to try to rule out battery issue because it sure sounds like there's an issue there whereby the system clock is momentarily left without power and resets to a default date/time (not unlike what would happen if a computer had no battery on is motherboard and the power supply was unplugged leaving the internal clock with no power to run).