What does "include the first day" mean?
From 1 Jan to 3 Jan the plain difference is 2 days, but counting the first day gives 3 (day 1, day 2, day 3). Hospital stays, campaign periods and similar durations are often counted this way.
Are public holidays excluded from business days?
No – only Saturdays and Sundays are excluded. Check for public holidays around the result if you need a strict count.
What happens when the resulting day does not exist (e.g. one month after 31 Jan)?
The result is clamped to the last day of that month (28 or 29 Feb). The same applies to years: one year after 29 Feb is 28 Feb.
How are years / months / days calculated?
The same way as an exact age: years first, then months, then days. Because months differ in length this can differ slightly from the plain day count.