I have 2 dates, 5/11/2010 and 6/16/2010 days between is 26 days. It doesn't seem to matter what flags I use for "inclusion". The last 2 arguments are used set to TRUE or FALSE depending on the endpoints you want to include. I've attached a real simple spreadsheet to show what I'm looking at. The field in yellow are my dates. The cells in green are my results. Can anybody tell me if I'm doing something wrong?
If this still happens with the latest release, please file an issue at https://github.com/eehlers/QuantLibXL.