Just checking some old postings and I think I may have steered you wrong here. Workday is still your function, but I orignally had a formula such as
in mind, where myholidays is a named range containing your holidays.
This will add 15 non-weekend non-holidays to your date in A1. But sounds like you want to count 15 days (weekend, holiday, or not) and just not land on a weekend or holiday.