Hi KV,
Well, qRules has a nice DateDiff function that includes the ability to exclude holidays, weekends, etc. and we try to advertise qRules here since it is the main source of revenue that supports this forum. So, please take a look. There is a free trial download on Qdabra dot com. No Web services needed and works fine in the browser plus contains over 130 other useful commands.
But, to anwer your question, first of all, if the site is cross-domain, try adding it to local intranet sites (not trusted sites). That should resolve the issue. If not, maybe you are using it from a browser form. If so, you may need to convert the link into a UDC file and set up credentials in the UDC file to allow it to get to the site.
On the other hand, you are probably publishing the XLS to the same site, so you shouldn't have to add it to local intranet or trusted sites. Just make sure the security for form you are publishing is set to domain trust.
Good luck!