06-21-2014 07:38 PM
Remove the battery for a couple days.
The (!) note on p. 30 of the manual as much as says that. Like most computers and increasingly more DSLRs, the date/time back-up battery is a cell permanently embedded on its circuit board that recharges from the main battery. It's only intended to hold settings for a short time between battery changes or short term storage. It won't hold settings for long term storage like the button battery cells