No, it shouldn't. If you take any two numbers, you will get the same result. 12 - 11 = 1. It doesn't matter if it is two dates, or the number of beans in two tins.
If you want the number to be inclusive of partial days, you will have to add the one on manually:
TimeSpan ts = endDate.Subtract(startDate);
txtTotalDays.Text = (ts.Days + 1).ToString();