Terrible idea -- BAC can vary between people, users may fudge timelines. And how soon is it until someone gets a DUI and blames it on the app telling them they were right under the limit?
Like any system, being only as accurate as the data provided to it, that's a given. All we can do is encourage users to input as accurately as possible and make it simple for them to do so.
I'm sure you're aware that not every app out there is 'serious'. Of course, we'll make users aware that this should be no substitute for common sense (i.e. not having any alcohol before driving).