How it works
Today is taken from your device, so the answer is right for your time zone. Days and weeks are added as they are; months and years follow the calendar, and when the day does not exist in the target month, the result is the last day of that month.
Business days skip Saturdays and Sundays. Public holidays are not skipped because they differ by country; add yours in the business days calculator. To start from a date other than today, use add days to a date.