Which one of these sentences is correct, or is either of them correct?
I eat three tablespoons of molasses at most on a day.
I eat three tablespoons of molasses at most in a day.
Context: On a given day, the speaker doesn’t eat more than three tablespoons of it if he prefers to eat molasses that day. I am asking this because I am familiar with the idiomatic usage of “in one day,” but “in a day” sounded sort of off to me. Is “on a day” the only correct usage here?