Excel (2010) Function Issue

I have a FUNCTION issue with Excel 2010. The attached screenshot is what I am experiencing. I’m trying to get the difference (quantity) between a date range. As you can see in this screenshot, the amount of days between Sept. 12 and Sept. 19 is 7 days, not 6, as it’s showing (?). Has anyone ever seen anything like there (when an Excel function is inaccurate)????