Here's what I think the problem is. Well on that day you changed the battery your tama's age must of been zero next day he turned 1 and this day he should be 2.
Go to google and type in debugging tama it'll show you but if you debug your tama there's a risk your tama can suffer from a series of glitches and even death when you debug the life cycle goes faster making it age that's why you need to rebug it quickly or of course it will die. So you have 2 choices