How it's calculated
days = end date − start date
difference in whole days between the two dates; also shown as weeks (days ÷ 7) and years (days ÷ 365.25).
Common questions
Does the count include both the start and end dates?
By default it counts the gap, so the start day isn't counted twice. Switch to inclusive mode if you need both endpoints in the total.
What counts as a business day?
Monday through Friday. Weekends are dropped, though public holidays aren't unless you exclude them separately.