Days in the US

I've been trying to get information on a couple of questions, tried the US Government website but the answer to my question came back as I had posted it under others (I couldn't find a category that it seem to fit) I did not get an answer. I wonder if anyone on this forum has asked a similar question and gotten an answer. The first question was about what constitutes a "day" in the US, we often go across for fuel or spend the night leaving the next morning. I'm not sure if the fuel trip is a day or if we have to count the day we leave after spending the night in the US as a day, anyone know for sure? The other question was I wanted to know if transit through the US from one destination in Canada to another in Canada counted, I believe I read somewhere that days in transit to Mexico were not considered days in the states.