Power off... maybe.
lol, recently I had a brilliant idea: went for a week to a place where no plug was to be found to recharge (and spare battery was shite).
With my mobile off overnight I found out with some shock that the battery went down overnight, something like 20% (and I didn't leave it out in the cold). Very sneaky, I thought, so it still runs, even if it doesn. So next night I took the battery out, smartass. No further loss of battery life, hurrah. And the next night. Only that on that next night's morning, when the battery went back in, the phone told me it's 1 Jan 2000, and couldn't find any networks for a very, very long time. Not, that it mattered, but left me scratching my head: why can't they save some basic data to a backup memory... I guess a matter of extra couple of cents to fit some tiny extra battery...