I found an unopened box of chocolate chip bread mix however the "best use by this date" is over a year old however I don't really understand how it could go bad. Would it be safe to bake it or should I just throw it out.
It depends on the ingredients in them. And it is the ingredients that potency changes over time. Yeast over long periods develops spores. Baking soda looses it's effectiveness, etc. You should always check the expiration dates on any boxed foods that have been on the shelf too long.
if it is over a year then throw it out. i do not understand it either but i did the mistake of baking with a 4 month expired box of cake mix and it did not go well. i would say within a week and some cases a month of the expiration date it is still good