just to be (swiss timing) precise: UA counts 23 hours 59 minutes not yet as a stopover - 24 hours 00 minutes is considered a stopover.

That's UA's rule for ticketing, for luggage (if you have to check it in again on a overnight stay, or not) other rules apply (luggage example: if you take the very next flight with UA next morning, than only you can check the luggage trough.