Why does it matter whether it's 10 or 11? I mean, seriously, who cares?

When I'm counting up the days, I generally count both travel days. When I'm gone Monday through Thursday for a typical work trip, I think of it as 4 days, even if the Thursday bit gets back home midday.

The only time this even remotely matters is if you have one of those antiquated expense reimbursement systems that has the concept "dayparts". I remember one of those from a client I worked on in the 90's, but haven't seen that concept since. (In that system, I literally had to record the exact time I left my house and returned to my house before/after a trip. It affected the per diems.)