You only count the days for which you spent the entire day/night outside the United States - hence the full 24 hour rule. So your departure date counts as being away - regardless of when you left - because obviously you would spend the next 24 hours out of the US. But your arrival date does not count towards the absence. For example, a day trip to Canada should not be listed.
The guide states:
"Write the number of trips you have taken outside the United States during the last five (or three) years. Count every trip that lasted 24 hours or longer."
Thanks for your clarification on this matter, I appreciate it!