Pick two dates to see the gap in days, weeks, months and years, with a separate business-day count that skips weekends. The second mode goes the other way: give it a date and a number of days to add or subtract and it tells you where you land.
The plain difference is exclusive: 1 March to 3 March is 2 days. An inclusive count, which is what you usually want for booking nights or project durations, is shown alongside it as 3 days.
No, only Saturdays and Sundays. Public holidays vary by country and region, so subtract those yourself for your jurisdiction.