The three questions
How long between two dates gives you the span in calendar years, months and days, plus the raw day count, weeks and working days. Both are useful for different things: a contract cares about calendar months, a project plan cares about days.
What date is X from a date counts forward or backward. Use a negative number to go back. Days, weeks, months and years are all available, and months behave the way a calendar does rather than as a fixed number of days.
How many working days counts Monday to Friday between two dates, and shows how many weekend days were skipped.
Why the end date is a choice
Ask two people how many days are in “1 March to 3 March” and you will get 2 and 3. Both are right, for different questions. A hotel stay is two nights. A three-day event is three days.
Rather than pick one and be wrong for half of visitors, the checkbox makes it explicit. Notice periods and deadlines in contracts usually count the end date; durations and stays usually do not.
Working days, honestly
The working-day count excludes Saturdays and Sundays and nothing else. Public holidays are not excluded, and they cannot sensibly be: they differ by country, and within Germany, Switzerland and Austria they differ by region, so a fixed list would produce a confidently wrong answer for most people.
If your deadline depends on holidays, use this number as the upper bound and subtract the holidays you know about.
Months are not 30 days
Adding one month to the 31st of January gives the 28th of February, because the 31st does not exist there. Add another month and you get the 28th of March, not the 31st. That is how calendars work and how contracts are read, and it is why “add 60 days” and “add 2 months” give different answers.
Questions
Is the end date counted?
Only if you tick the box. That single choice is the most common source of off-by-one disagreements, so it is a visible option rather than a silent assumption.
Are public holidays excluded from working days?
No, only Saturdays and Sundays. Public holidays differ by country and by region within a country, so excluding a fixed set would be wrong for most visitors.
How are months counted?
Calendar-wise, the way people count them: from the 31st of January, one month later is the 28th of February, because there is no 31st. Adding a month twice is therefore not always the same as adding two months.
Does daylight saving affect the result?
No. Dates are handled at UTC midnight internally, so a clock change cannot turn 30 days into 29 days and 23 hours.
Why do years, months and days not add up to the day count?
They are two different ways of expressing the same span. “1 year, 2 months” depends on which months those were; the day count does not.