Yeah, when the batteries die in these games, the time based events, like growing berries or evolving Eevee into Umbreon/Espeon, no longer work. It's just stuck, but the game can still be played and saved unlike when batteries die in the older games. You can replace them I'm sure, but I really don't understand why anyone would bother at this point. Berries aren't that important. But perhaps Google replacing the battery in gen III games or even checking YouTube. I'm sure there are video tutorials, though of course buying a battery will be on your own time and money. Honestly though, if I were you, I just wouldn't worry about it. Berries aren't a huge loss, and as much as I love these games, it's two generations behind. So getting a lot of berries in gen III doesn't matter too much.