Currently, the WORKDAY(start_date,days) function is off by 1 day. For example, WORKDAY(start_date,1) should add one day to start_date, but it is returning the start_date. For compatibility with Google Sheets and Excel, this function should add n work day to the start_date. This is an issue with WORKDAY.INTL as well.
In addition, the days argument should be allowed to be negative (so that you can subtract a number of work days from the start date). Currently, this is resulting in a #NUM error.
These functions are used heavily in project management.