How Date difference works
Date difference measures the full gap between two calendar dates, in years, months and days, plus totals like total days and weeks. It counts every day in the range, which is what you want for ages, durations and "how long between" questions.
If you only care about working days, the Business days calculator excludes weekends (and holidays you add); if one end is always today, Days until a date or Time since a date are quicker. The totals are shown explicitly, so you can see exactly whether the last day is included in the count.