Errrmmmm, perhaps it helps because it’s the answer to the OP’s question. It is authoritative definition of what ‘day’ means when that word is used to mean a time interval.
Yeah but it doesn't define when it starts and stops, which is probably what the OP really wants to know. (I'd work on midnight to midnight, but it's always nice to find that sort of thing in writing, particularly if your company is trying to screw you over.)