Baking Powder does not really go bad and can last basically forever.
But when baking powder sits out for longer than a year then it may become hard but that does not mean that it's bad or unsafe to eat.
Baking powder remains safe to use for a very long time even after getting hard and if the baking powder does get hard all you need to do is mash it up with a meat tenderizer hammer or something else and break the baking powder back up.
Then you can use it in baking goods etc.
I've used baking powder that was sealed for 7 years in my mother's pantry when we were cleaning her house out to move.
She was gonna throw it out but I took it and used it.