How days are counted
The count is the difference between the two calendar dates, so the start day is not included and the end day is. From 1 January to 2 January is 1 day. Dates are handled as plain calendar days, so daylight saving time never adds or removes a day, and leap years are included.
Years, months and days
The "years, months, days" figure counts whole years first, then whole months, then the remaining days. When a month is shorter than the starting day, the last day of that month is used: one month after 31 January is 28 February (29 in a leap year).
Business days
Business days count Monday to Friday from the start date up to, but not including, the end date. Public holidays are not excluded here. To exclude your own holidays, use the business days calculator.