title: adding workdays to date taking into account weekends and holidays
body: <p>given a date, and a list of holidays, how do i add the given number of workdays to this date? there are a lot of solutions to the smaller problem which does not take account of holidays (see for example <a href="https://stackoverflow.com/questions/279296/adding-days-to-a-date-but-excluding-weekends">Adding Days to a Date but Excluding Weekends</a>).</p>
  
  <p>EDIT: i am looking for a O(1) or at least linear (on the number of holidays) solution.</p>
  
  <p>please help
  thanks
  konstantin</p>